[1/2] locking/lockdep: restore cross-release checks
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Message ID 20181024114412.31093-1-daniel.vetter@ffwll.ch
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  • [1/2] locking/lockdep: restore cross-release checks
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Daniel Vetter Oct. 24, 2018, 11:44 a.m. UTC
This reverts the following commits:

commit 527187d28569e39c5d489d6306d3b79605cf85a6
Author: Ingo Molnar <mingo@kernel.org>
Date:   Mon Jan 8 17:27:19 2018 +0100

    locking/lockdep: Remove cross-release leftovers

commit dba04eb76df982703fefc021a4d278347b6176a9
Author: David Sterba <dsterba@suse.com>
Date:   Mon Jan 8 16:27:31 2018 +0100

    locking/Documentation: Remove stale crossrelease_fullstack parameter

commit e966eaeeb623f09975ef362c2866fae6f86844f9
Author: Ingo Molnar <mingo@kernel.org>
Date:   Tue Dec 12 12:31:16 2017 +0100

    locking/lockdep: Remove the cross-release locking checks

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 .../admin-guide/kernel-parameters.txt         |   3 +
 Documentation/locking/crossrelease.txt        | 874 ++++++++++++++++++
 include/linux/completion.h                    |  46 +
 include/linux/irqflags.h                      |   4 +
 include/linux/lockdep.h                       | 127 +++
 include/linux/sched.h                         |  11 +
 kernel/locking/lockdep.c                      | 652 ++++++++++++-
 kernel/sched/completion.c                     |   5 +
 lib/Kconfig.debug                             |  33 +
 9 files changed, 1720 insertions(+), 35 deletions(-)
 create mode 100644 Documentation/locking/crossrelease.txt

Patch
diff mbox series

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 92eb1f42240d..6ab3ad188c01 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -724,6 +724,9 @@ 
 			It will be ignored when crashkernel=X,high is not used
 			or memory reserved is below 4G.
 
+	crossrelease_fullstack
+			[KNL] Allow to record full stack trace in cross-release
+
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests
 
diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt
new file mode 100644
index 000000000000..bdf1423d5f99
--- /dev/null
+++ b/Documentation/locking/crossrelease.txt
@@ -0,0 +1,874 @@ 
+Crossrelease
+============
+
+Started by Byungchul Park <byungchul.park@lge.com>
+
+Contents:
+
+ (*) Background
+
+     - What causes deadlock
+     - How lockdep works
+
+ (*) Limitation
+
+     - Limit lockdep
+     - Pros from the limitation
+     - Cons from the limitation
+     - Relax the limitation
+
+ (*) Crossrelease
+
+     - Introduce crossrelease
+     - Introduce commit
+
+ (*) Implementation
+
+     - Data structures
+     - How crossrelease works
+
+ (*) Optimizations
+
+     - Avoid duplication
+     - Lockless for hot paths
+
+ (*) APPENDIX A: What lockdep does to work aggresively
+
+ (*) APPENDIX B: How to avoid adding false dependencies
+
+
+==========
+Background
+==========
+
+What causes deadlock
+--------------------
+
+A deadlock occurs when a context is waiting for an event to happen,
+which is impossible because another (or the) context who can trigger the
+event is also waiting for another (or the) event to happen, which is
+also impossible due to the same reason.
+
+For example:
+
+   A context going to trigger event C is waiting for event A to happen.
+   A context going to trigger event A is waiting for event B to happen.
+   A context going to trigger event B is waiting for event C to happen.
+
+A deadlock occurs when these three wait operations run at the same time,
+because event C cannot be triggered if event A does not happen, which in
+turn cannot be triggered if event B does not happen, which in turn
+cannot be triggered if event C does not happen. After all, no event can
+be triggered since any of them never meets its condition to wake up.
+
+A dependency might exist between two waiters and a deadlock might happen
+due to an incorrect releationship between dependencies. Thus, we must
+define what a dependency is first. A dependency exists between them if:
+
+   1. There are two waiters waiting for each event at a given time.
+   2. The only way to wake up each waiter is to trigger its event.
+   3. Whether one can be woken up depends on whether the other can.
+
+Each wait in the example creates its dependency like:
+
+   Event C depends on event A.
+   Event A depends on event B.
+   Event B depends on event C.
+
+   NOTE: Precisely speaking, a dependency is one between whether a
+   waiter for an event can be woken up and whether another waiter for
+   another event can be woken up. However from now on, we will describe
+   a dependency as if it's one between an event and another event for
+   simplicity.
+
+And they form circular dependencies like:
+
+    -> C -> A -> B -
+   /                \
+   \                /
+    ----------------
+
+   where 'A -> B' means that event A depends on event B.
+
+Such circular dependencies lead to a deadlock since no waiter can meet
+its condition to wake up as described.
+
+CONCLUSION
+
+Circular dependencies cause a deadlock.
+
+
+How lockdep works
+-----------------
+
+Lockdep tries to detect a deadlock by checking dependencies created by
+lock operations, acquire and release. Waiting for a lock corresponds to
+waiting for an event, and releasing a lock corresponds to triggering an
+event in the previous section.
+
+In short, lockdep does:
+
+   1. Detect a new dependency.
+   2. Add the dependency into a global graph.
+   3. Check if that makes dependencies circular.
+   4. Report a deadlock or its possibility if so.
+
+For example, consider a graph built by lockdep that looks like:
+
+   A -> B -
+           \
+            -> E
+           /
+   C -> D -
+
+   where A, B,..., E are different lock classes.
+
+Lockdep will add a dependency into the graph on detection of a new
+dependency. For example, it will add a dependency 'E -> C' when a new
+dependency between lock E and lock C is detected. Then the graph will be:
+
+       A -> B -
+               \
+                -> E -
+               /      \
+    -> C -> D -        \
+   /                   /
+   \                  /
+    ------------------
+
+   where A, B,..., E are different lock classes.
+
+This graph contains a subgraph which demonstrates circular dependencies:
+
+                -> E -
+               /      \
+    -> C -> D -        \
+   /                   /
+   \                  /
+    ------------------
+
+   where C, D and E are different lock classes.
+
+This is the condition under which a deadlock might occur. Lockdep
+reports it on detection after adding a new dependency. This is the way
+how lockdep works.
+
+CONCLUSION
+
+Lockdep detects a deadlock or its possibility by checking if circular
+dependencies were created after adding each new dependency.
+
+
+==========
+Limitation
+==========
+
+Limit lockdep
+-------------
+
+Limiting lockdep to work on only typical locks e.g. spin locks and
+mutexes, which are released within the acquire context, the
+implementation becomes simple but its capacity for detection becomes
+limited. Let's check pros and cons in next section.
+
+
+Pros from the limitation
+------------------------
+
+Given the limitation, when acquiring a lock, locks in a held_locks
+cannot be released if the context cannot acquire it so has to wait to
+acquire it, which means all waiters for the locks in the held_locks are
+stuck. It's an exact case to create dependencies between each lock in
+the held_locks and the lock to acquire.
+
+For example:
+
+   CONTEXT X
+   ---------
+   acquire A
+   acquire B /* Add a dependency 'A -> B' */
+   release B
+   release A
+
+   where A and B are different lock classes.
+
+When acquiring lock A, the held_locks of CONTEXT X is empty thus no
+dependency is added. But when acquiring lock B, lockdep detects and adds
+a new dependency 'A -> B' between lock A in the held_locks and lock B.
+They can be simply added whenever acquiring each lock.
+
+And data required by lockdep exists in a local structure, held_locks
+embedded in task_struct. Forcing to access the data within the context,
+lockdep can avoid racy problems without explicit locks while handling
+the local data.
+
+Lastly, lockdep only needs to keep locks currently being held, to build
+a dependency graph. However, relaxing the limitation, it needs to keep
+even locks already released, because a decision whether they created
+dependencies might be long-deferred.
+
+To sum up, we can expect several advantages from the limitation:
+
+   1. Lockdep can easily identify a dependency when acquiring a lock.
+   2. Races are avoidable while accessing local locks in a held_locks.
+   3. Lockdep only needs to keep locks currently being held.
+
+CONCLUSION
+
+Given the limitation, the implementation becomes simple and efficient.
+
+
+Cons from the limitation
+------------------------
+
+Given the limitation, lockdep is applicable only to typical locks. For
+example, page locks for page access or completions for synchronization
+cannot work with lockdep.
+
+Can we detect deadlocks below, under the limitation?
+
+Example 1:
+
+   CONTEXT X	   CONTEXT Y	   CONTEXT Z
+   ---------	   ---------	   ----------
+		   mutex_lock A
+   lock_page B
+		   lock_page B
+				   mutex_lock A /* DEADLOCK */
+				   unlock_page B held by X
+		   unlock_page B
+		   mutex_unlock A
+				   mutex_unlock A
+
+   where A and B are different lock classes.
+
+No, we cannot.
+
+Example 2:
+
+   CONTEXT X		   CONTEXT Y
+   ---------		   ---------
+			   mutex_lock A
+   mutex_lock A
+			   wait_for_complete B /* DEADLOCK */
+   complete B
+			   mutex_unlock A
+   mutex_unlock A
+
+   where A is a lock class and B is a completion variable.
+
+No, we cannot.
+
+CONCLUSION
+
+Given the limitation, lockdep cannot detect a deadlock or its
+possibility caused by page locks or completions.
+
+
+Relax the limitation
+--------------------
+
+Under the limitation, things to create dependencies are limited to
+typical locks. However, synchronization primitives like page locks and
+completions, which are allowed to be released in any context, also
+create dependencies and can cause a deadlock. So lockdep should track
+these locks to do a better job. We have to relax the limitation for
+these locks to work with lockdep.
+
+Detecting dependencies is very important for lockdep to work because
+adding a dependency means adding an opportunity to check whether it
+causes a deadlock. The more lockdep adds dependencies, the more it
+thoroughly works. Thus Lockdep has to do its best to detect and add as
+many true dependencies into a graph as possible.
+
+For example, considering only typical locks, lockdep builds a graph like:
+
+   A -> B -
+           \
+            -> E
+           /
+   C -> D -
+
+   where A, B,..., E are different lock classes.
+
+On the other hand, under the relaxation, additional dependencies might
+be created and added. Assuming additional 'FX -> C' and 'E -> GX' are
+added thanks to the relaxation, the graph will be:
+
+         A -> B -
+                 \
+                  -> E -> GX
+                 /
+   FX -> C -> D -
+
+   where A, B,..., E, FX and GX are different lock classes, and a suffix
+   'X' is added on non-typical locks.
+
+The latter graph gives us more chances to check circular dependencies
+than the former. However, it might suffer performance degradation since
+relaxing the limitation, with which design and implementation of lockdep
+can be efficient, might introduce inefficiency inevitably. So lockdep
+should provide two options, strong detection and efficient detection.
+
+Choosing efficient detection:
+
+   Lockdep works with only locks restricted to be released within the
+   acquire context. However, lockdep works efficiently.
+
+Choosing strong detection:
+
+   Lockdep works with all synchronization primitives. However, lockdep
+   suffers performance degradation.
+
+CONCLUSION
+
+Relaxing the limitation, lockdep can add additional dependencies giving
+additional opportunities to check circular dependencies.
+
+
+============
+Crossrelease
+============
+
+Introduce crossrelease
+----------------------
+
+In order to allow lockdep to handle additional dependencies by what
+might be released in any context, namely 'crosslock', we have to be able
+to identify those created by crosslocks. The proposed 'crossrelease'
+feature provoides a way to do that.
+
+Crossrelease feature has to do:
+
+   1. Identify dependencies created by crosslocks.
+   2. Add the dependencies into a dependency graph.
+
+That's all. Once a meaningful dependency is added into graph, then
+lockdep would work with the graph as it did. The most important thing
+crossrelease feature has to do is to correctly identify and add true
+dependencies into the global graph.
+
+A dependency e.g. 'A -> B' can be identified only in the A's release
+context because a decision required to identify the dependency can be
+made only in the release context. That is to decide whether A can be
+released so that a waiter for A can be woken up. It cannot be made in
+other than the A's release context.
+
+It's no matter for typical locks because each acquire context is same as
+its release context, thus lockdep can decide whether a lock can be
+released in the acquire context. However for crosslocks, lockdep cannot
+make the decision in the acquire context but has to wait until the
+release context is identified.
+
+Therefore, deadlocks by crosslocks cannot be detected just when it
+happens, because those cannot be identified until the crosslocks are
+released. However, deadlock possibilities can be detected and it's very
+worth. See 'APPENDIX A' section to check why.
+
+CONCLUSION
+
+Using crossrelease feature, lockdep can work with what might be released
+in any context, namely crosslock.
+
+
+Introduce commit
+----------------
+
+Since crossrelease defers the work adding true dependencies of
+crosslocks until they are actually released, crossrelease has to queue
+all acquisitions which might create dependencies with the crosslocks.
+Then it identifies dependencies using the queued data in batches at a
+proper time. We call it 'commit'.
+
+There are four types of dependencies:
+
+1. TT type: 'typical lock A -> typical lock B'
+
+   Just when acquiring B, lockdep can see it's in the A's release
+   context. So the dependency between A and B can be identified
+   immediately. Commit is unnecessary.
+
+2. TC type: 'typical lock A -> crosslock BX'
+
+   Just when acquiring BX, lockdep can see it's in the A's release
+   context. So the dependency between A and BX can be identified
+   immediately. Commit is unnecessary, too.
+
+3. CT type: 'crosslock AX -> typical lock B'
+
+   When acquiring B, lockdep cannot identify the dependency because
+   there's no way to know if it's in the AX's release context. It has
+   to wait until the decision can be made. Commit is necessary.
+
+4. CC type: 'crosslock AX -> crosslock BX'
+
+   When acquiring BX, lockdep cannot identify the dependency because
+   there's no way to know if it's in the AX's release context. It has
+   to wait until the decision can be made. Commit is necessary.
+   But, handling CC type is not implemented yet. It's a future work.
+
+Lockdep can work without commit for typical locks, but commit step is
+necessary once crosslocks are involved. Introducing commit, lockdep
+performs three steps. What lockdep does in each step is:
+
+1. Acquisition: For typical locks, lockdep does what it originally did
+   and queues the lock so that CT type dependencies can be checked using
+   it at the commit step. For crosslocks, it saves data which will be
+   used at the commit step and increases a reference count for it.
+
+2. Commit: No action is reauired for typical locks. For crosslocks,
+   lockdep adds CT type dependencies using the data saved at the
+   acquisition step.
+
+3. Release: No changes are required for typical locks. When a crosslock
+   is released, it decreases a reference count for it.
+
+CONCLUSION
+
+Crossrelease introduces commit step to handle dependencies of crosslocks
+in batches at a proper time.
+
+
+==============
+Implementation
+==============
+
+Data structures
+---------------
+
+Crossrelease introduces two main data structures.
+
+1. hist_lock
+
+   This is an array embedded in task_struct, for keeping lock history so
+   that dependencies can be added using them at the commit step. Since
+   it's local data, it can be accessed locklessly in the owner context.
+   The array is filled at the acquisition step and consumed at the
+   commit step. And it's managed in circular manner.
+
+2. cross_lock
+
+   One per lockdep_map exists. This is for keeping data of crosslocks
+   and used at the commit step.
+
+
+How crossrelease works
+----------------------
+
+It's the key of how crossrelease works, to defer necessary works to an
+appropriate point in time and perform in at once at the commit step.
+Let's take a look with examples step by step, starting from how lockdep
+works without crossrelease for typical locks.
+
+   acquire A /* Push A onto held_locks */
+   acquire B /* Push B onto held_locks and add 'A -> B' */
+   acquire C /* Push C onto held_locks and add 'B -> C' */
+   release C /* Pop C from held_locks */
+   release B /* Pop B from held_locks */
+   release A /* Pop A from held_locks */
+
+   where A, B and C are different lock classes.
+
+   NOTE: This document assumes that readers already understand how
+   lockdep works without crossrelease thus omits details. But there's
+   one thing to note. Lockdep pretends to pop a lock from held_locks
+   when releasing it. But it's subtly different from the original pop
+   operation because lockdep allows other than the top to be poped.
+
+In this case, lockdep adds 'the top of held_locks -> the lock to acquire'
+dependency every time acquiring a lock.
+
+After adding 'A -> B', a dependency graph will be:
+
+   A -> B
+
+   where A and B are different lock classes.
+
+And after adding 'B -> C', the graph will be:
+
+   A -> B -> C
+
+   where A, B and C are different lock classes.
+
+Let's performs commit step even for typical locks to add dependencies.
+Of course, commit step is not necessary for them, however, it would work
+well because this is a more general way.
+
+   acquire A
+   /*
+    * Queue A into hist_locks
+    *
+    * In hist_locks: A
+    * In graph: Empty
+    */
+
+   acquire B
+   /*
+    * Queue B into hist_locks
+    *
+    * In hist_locks: A, B
+    * In graph: Empty
+    */
+
+   acquire C
+   /*
+    * Queue C into hist_locks
+    *
+    * In hist_locks: A, B, C
+    * In graph: Empty
+    */
+
+   commit C
+   /*
+    * Add 'C -> ?'
+    * Answer the following to decide '?'
+    * What has been queued since acquire C: Nothing
+    *
+    * In hist_locks: A, B, C
+    * In graph: Empty
+    */
+
+   release C
+
+   commit B
+   /*
+    * Add 'B -> ?'
+    * Answer the following to decide '?'
+    * What has been queued since acquire B: C
+    *
+    * In hist_locks: A, B, C
+    * In graph: 'B -> C'
+    */
+
+   release B
+
+   commit A
+   /*
+    * Add 'A -> ?'
+    * Answer the following to decide '?'
+    * What has been queued since acquire A: B, C
+    *
+    * In hist_locks: A, B, C
+    * In graph: 'B -> C', 'A -> B', 'A -> C'
+    */
+
+   release A
+
+   where A, B and C are different lock classes.
+
+In this case, dependencies are added at the commit step as described.
+
+After commits for A, B and C, the graph will be:
+
+   A -> B -> C
+
+   where A, B and C are different lock classes.
+
+   NOTE: A dependency 'A -> C' is optimized out.
+
+We can see the former graph built without commit step is same as the
+latter graph built using commit steps. Of course the former way leads to
+earlier finish for building the graph, which means we can detect a
+deadlock or its possibility sooner. So the former way would be prefered
+when possible. But we cannot avoid using the latter way for crosslocks.
+
+Let's look at how commit steps work for crosslocks. In this case, the
+commit step is performed only on crosslock AX as real. And it assumes
+that the AX release context is different from the AX acquire context.
+
+   BX RELEASE CONTEXT		   BX ACQUIRE CONTEXT
+   ------------------		   ------------------
+				   acquire A
+				   /*
+				    * Push A onto held_locks
+				    * Queue A into hist_locks
+				    *
+				    * In held_locks: A
+				    * In hist_locks: A
+				    * In graph: Empty
+				    */
+
+				   acquire BX
+				   /*
+				    * Add 'the top of held_locks -> BX'
+				    *
+				    * In held_locks: A
+				    * In hist_locks: A
+				    * In graph: 'A -> BX'
+				    */
+
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   It must be guaranteed that the following operations are seen after
+   acquiring BX globally. It can be done by things like barrier.
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+   acquire C
+   /*
+    * Push C onto held_locks
+    * Queue C into hist_locks
+    *
+    * In held_locks: C
+    * In hist_locks: C
+    * In graph: 'A -> BX'
+    */
+
+   release C
+   /*
+    * Pop C from held_locks
+    *
+    * In held_locks: Empty
+    * In hist_locks: C
+    * In graph: 'A -> BX'
+    */
+				   acquire D
+				   /*
+				    * Push D onto held_locks
+				    * Queue D into hist_locks
+				    * Add 'the top of held_locks -> D'
+				    *
+				    * In held_locks: A, D
+				    * In hist_locks: A, D
+				    * In graph: 'A -> BX', 'A -> D'
+				    */
+   acquire E
+   /*
+    * Push E onto held_locks
+    * Queue E into hist_locks
+    *
+    * In held_locks: E
+    * In hist_locks: C, E
+    * In graph: 'A -> BX', 'A -> D'
+    */
+
+   release E
+   /*
+    * Pop E from held_locks
+    *
+    * In held_locks: Empty
+    * In hist_locks: D, E
+    * In graph: 'A -> BX', 'A -> D'
+    */
+				   release D
+				   /*
+				    * Pop D from held_locks
+				    *
+				    * In held_locks: A
+				    * In hist_locks: A, D
+				    * In graph: 'A -> BX', 'A -> D'
+				    */
+   commit BX
+   /*
+    * Add 'BX -> ?'
+    * What has been queued since acquire BX: C, E
+    *
+    * In held_locks: Empty
+    * In hist_locks: D, E
+    * In graph: 'A -> BX', 'A -> D',
+    *           'BX -> C', 'BX -> E'
+    */
+
+   release BX
+   /*
+    * In held_locks: Empty
+    * In hist_locks: D, E
+    * In graph: 'A -> BX', 'A -> D',
+    *           'BX -> C', 'BX -> E'
+    */
+				   release A
+				   /*
+				    * Pop A from held_locks
+				    *
+				    * In held_locks: Empty
+				    * In hist_locks: A, D
+				    * In graph: 'A -> BX', 'A -> D',
+				    *           'BX -> C', 'BX -> E'
+				    */
+
+   where A, BX, C,..., E are different lock classes, and a suffix 'X' is
+   added on crosslocks.
+
+Crossrelease considers all acquisitions after acqiuring BX are
+candidates which might create dependencies with BX. True dependencies
+will be determined when identifying the release context of BX. Meanwhile,
+all typical locks are queued so that they can be used at the commit step.
+And then two dependencies 'BX -> C' and 'BX -> E' are added at the
+commit step when identifying the release context.
+
+The final graph will be, with crossrelease:
+
+               -> C
+              /
+       -> BX -
+      /       \
+   A -         -> E
+      \
+       -> D
+
+   where A, BX, C,..., E are different lock classes, and a suffix 'X' is
+   added on crosslocks.
+
+However, the final graph will be, without crossrelease:
+
+   A -> D
+
+   where A and D are different lock classes.
+
+The former graph has three more dependencies, 'A -> BX', 'BX -> C' and
+'BX -> E' giving additional opportunities to check if they cause
+deadlocks. This way lockdep can detect a deadlock or its possibility
+caused by crosslocks.
+
+CONCLUSION
+
+We checked how crossrelease works with several examples.
+
+
+=============
+Optimizations
+=============
+
+Avoid duplication
+-----------------
+
+Crossrelease feature uses a cache like what lockdep already uses for
+dependency chains, but this time it's for caching CT type dependencies.
+Once that dependency is cached, the same will never be added again.
+
+
+Lockless for hot paths
+----------------------
+
+To keep all locks for later use at the commit step, crossrelease adopts
+a local array embedded in task_struct, which makes access to the data
+lockless by forcing it to happen only within the owner context. It's
+like how lockdep handles held_locks. Lockless implmentation is important
+since typical locks are very frequently acquired and released.
+
+
+=================================================
+APPENDIX A: What lockdep does to work aggresively
+=================================================
+
+A deadlock actually occurs when all wait operations creating circular
+dependencies run at the same time. Even though they don't, a potential
+deadlock exists if the problematic dependencies exist. Thus it's
+meaningful to detect not only an actual deadlock but also its potential
+possibility. The latter is rather valuable. When a deadlock occurs
+actually, we can identify what happens in the system by some means or
+other even without lockdep. However, there's no way to detect possiblity
+without lockdep unless the whole code is parsed in head. It's terrible.
+Lockdep does the both, and crossrelease only focuses on the latter.
+
+Whether or not a deadlock actually occurs depends on several factors.
+For example, what order contexts are switched in is a factor. Assuming
+circular dependencies exist, a deadlock would occur when contexts are
+switched so that all wait operations creating the dependencies run
+simultaneously. Thus to detect a deadlock possibility even in the case
+that it has not occured yet, lockdep should consider all possible
+combinations of dependencies, trying to:
+
+1. Use a global dependency graph.
+
+   Lockdep combines all dependencies into one global graph and uses them,
+   regardless of which context generates them or what order contexts are
+   switched in. Aggregated dependencies are only considered so they are
+   prone to be circular if a problem exists.
+
+2. Check dependencies between classes instead of instances.
+
+   What actually causes a deadlock are instances of lock. However,
+   lockdep checks dependencies between classes instead of instances.
+   This way lockdep can detect a deadlock which has not happened but
+   might happen in future by others but the same class.
+
+3. Assume all acquisitions lead to waiting.
+
+   Although locks might be acquired without waiting which is essential
+   to create dependencies, lockdep assumes all acquisitions lead to
+   waiting since it might be true some time or another.
+
+CONCLUSION
+
+Lockdep detects not only an actual deadlock but also its possibility,
+and the latter is more valuable.
+
+
+==================================================
+APPENDIX B: How to avoid adding false dependencies
+==================================================
+
+Remind what a dependency is. A dependency exists if:
+
+   1. There are two waiters waiting for each event at a given time.
+   2. The only way to wake up each waiter is to trigger its event.
+   3. Whether one can be woken up depends on whether the other can.
+
+For example:
+
+   acquire A
+   acquire B /* A dependency 'A -> B' exists */
+   release B
+   release A
+
+   where A and B are different lock classes.
+
+A depedency 'A -> B' exists since:
+
+   1. A waiter for A and a waiter for B might exist when acquiring B.
+   2. Only way to wake up each is to release what it waits for.
+   3. Whether the waiter for A can be woken up depends on whether the
+      other can. IOW, TASK X cannot release A if it fails to acquire B.
+
+For another example:
+
+   TASK X			   TASK Y
+   ------			   ------
+				   acquire AX
+   acquire B /* A dependency 'AX -> B' exists */
+   release B
+   release AX held by Y
+
+   where AX and B are different lock classes, and a suffix 'X' is added
+   on crosslocks.
+
+Even in this case involving crosslocks, the same rule can be applied. A
+depedency 'AX -> B' exists since:
+
+   1. A waiter for AX and a waiter for B might exist when acquiring B.
+   2. Only way to wake up each is to release what it waits for.
+   3. Whether the waiter for AX can be woken up depends on whether the
+      other can. IOW, TASK X cannot release AX if it fails to acquire B.
+
+Let's take a look at more complicated example:
+
+   TASK X			   TASK Y
+   ------			   ------
+   acquire B
+   release B
+   fork Y
+				   acquire AX
+   acquire C /* A dependency 'AX -> C' exists */
+   release C
+   release AX held by Y
+
+   where AX, B and C are different lock classes, and a suffix 'X' is
+   added on crosslocks.
+
+Does a dependency 'AX -> B' exist? Nope.
+
+Two waiters are essential to create a dependency. However, waiters for
+AX and B to create 'AX -> B' cannot exist at the same time in this
+example. Thus the dependency 'AX -> B' cannot be created.
+
+It would be ideal if the full set of true ones can be considered. But
+we can ensure nothing but what actually happened. Relying on what
+actually happens at runtime, we can anyway add only true ones, though
+they might be a subset of true ones. It's similar to how lockdep works
+for typical locks. There might be more true dependencies than what
+lockdep has detected in runtime. Lockdep has no choice but to rely on
+what actually happens. Crossrelease also relies on it.
+
+CONCLUSION
+
+Relying on what actually happens, lockdep can avoid adding false
+dependencies.
diff --git a/include/linux/completion.h b/include/linux/completion.h
index 519e94915d18..0662a417febe 100644
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -10,6 +10,9 @@ 
  */
 
 #include <linux/wait.h>
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+#include <linux/lockdep.h>
+#endif
 
 /*
  * struct completion - structure used to maintain state for a "completion"
@@ -26,15 +29,58 @@ 
 struct completion {
 	unsigned int done;
 	wait_queue_head_t wait;
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+	struct lockdep_map_cross map;
+#endif
 };
 
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+static inline void complete_acquire(struct completion *x)
+{
+	lock_acquire_exclusive((struct lockdep_map *)&x->map, 0, 0, NULL, _RET_IP_);
+}
+
+static inline void complete_release(struct completion *x)
+{
+	lock_release((struct lockdep_map *)&x->map, 0, _RET_IP_);
+}
+
+static inline void complete_release_commit(struct completion *x)
+{
+	lock_commit_crosslock((struct lockdep_map *)&x->map);
+}
+
+#define init_completion_map(x, m)					\
+do {									\
+	lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map,	\
+			(m)->name, (m)->key, 0);				\
+	__init_completion(x);						\
+} while (0)
+
+#define init_completion(x)						\
+do {									\
+	static struct lock_class_key __key;				\
+	lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map,	\
+			"(completion)" #x,				\
+			&__key, 0);					\
+	__init_completion(x);						\
+} while (0)
+#else
 #define init_completion_map(x, m) __init_completion(x)
 #define init_completion(x) __init_completion(x)
 static inline void complete_acquire(struct completion *x) {}
 static inline void complete_release(struct completion *x) {}
+static inline void complete_release_commit(struct completion *x) {}
+#endif
 
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+#define COMPLETION_INITIALIZER(work) \
+	{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait), \
+	STATIC_CROSS_LOCKDEP_MAP_INIT("(completion)" #work, &(work)) }
+#else
 #define COMPLETION_INITIALIZER(work) \
 	{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
+#endif
 
 #define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \
 	(*({ init_completion_map(&(work), &(map)); &(work); }))
diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 21619c92c377..bd1bda7248bf 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -38,18 +38,22 @@ 
 # define trace_hardirq_enter()			\
 do {						\
 	current->hardirq_context++;		\
+	crossrelease_hist_start(XHLOCK_HARD);	\
 } while (0)
 # define trace_hardirq_exit()			\
 do {						\
 	current->hardirq_context--;		\
+	crossrelease_hist_end(XHLOCK_HARD);	\
 } while (0)
 # define lockdep_softirq_enter()		\
 do {						\
 	current->softirq_context++;		\
+	crossrelease_hist_start(XHLOCK_SOFT);	\
 } while (0)
 # define lockdep_softirq_exit()			\
 do {						\
 	current->softirq_context--;		\
+	crossrelease_hist_end(XHLOCK_SOFT);	\
 } while (0)
 #else
 # define trace_hardirqs_on()		do { } while (0)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index b0d0b51c4d85..59b973ee4ce5 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -158,6 +158,12 @@  struct lockdep_map {
 	int				cpu;
 	unsigned long			ip;
 #endif
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+	/*
+	 * Whether it's a crosslock.
+	 */
+	int				cross;
+#endif
 };
 
 static inline void lockdep_copy_map(struct lockdep_map *to,
@@ -261,8 +267,95 @@  struct held_lock {
 	unsigned int hardirqs_off:1;
 	unsigned int references:12;					/* 32 bits */
 	unsigned int pin_count;
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+	/*
+	 * Generation id.
+	 *
+	 * A value of cross_gen_id will be stored when holding this,
+	 * which is globally increased whenever each crosslock is held.
+	 */
+	unsigned int gen_id;
+#endif
+};
+
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#define MAX_XHLOCK_TRACE_ENTRIES 5
+
+/*
+ * This is for keeping locks waiting for commit so that true dependencies
+ * can be added at commit step.
+ */
+struct hist_lock {
+	/*
+	 * Id for each entry in the ring buffer. This is used to
+	 * decide whether the ring buffer was overwritten or not.
+	 *
+	 * For example,
+	 *
+	 *           |<----------- hist_lock ring buffer size ------->|
+	 *           pppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiii
+	 * wrapped > iiiiiiiiiiiiiiiiiiiiiiiiiii.......................
+	 *
+	 *           where 'p' represents an acquisition in process
+	 *           context, 'i' represents an acquisition in irq
+	 *           context.
+	 *
+	 * In this example, the ring buffer was overwritten by
+	 * acquisitions in irq context, that should be detected on
+	 * rollback or commit.
+	 */
+	unsigned int hist_id;
+
+	/*
+	 * Seperate stack_trace data. This will be used at commit step.
+	 */
+	struct stack_trace	trace;
+	unsigned long		trace_entries[MAX_XHLOCK_TRACE_ENTRIES];
+
+	/*
+	 * Seperate hlock instance. This will be used at commit step.
+	 *
+	 * TODO: Use a smaller data structure containing only necessary
+	 * data. However, we should make lockdep code able to handle the
+	 * smaller one first.
+	 */
+	struct held_lock	hlock;
 };
 
+/*
+ * To initialize a lock as crosslock, lockdep_init_map_crosslock() should
+ * be called instead of lockdep_init_map().
+ */
+struct cross_lock {
+	/*
+	 * When more than one acquisition of crosslocks are overlapped,
+	 * we have to perform commit for them based on cross_gen_id of
+	 * the first acquisition, which allows us to add more true
+	 * dependencies.
+	 *
+	 * Moreover, when no acquisition of a crosslock is in progress,
+	 * we should not perform commit because the lock might not exist
+	 * any more, which might cause incorrect memory access. So we
+	 * have to track the number of acquisitions of a crosslock.
+	 */
+	int nr_acquire;
+
+	/*
+	 * Seperate hlock instance. This will be used at commit step.
+	 *
+	 * TODO: Use a smaller data structure containing only necessary
+	 * data. However, we should make lockdep code able to handle the
+	 * smaller one first.
+	 */
+	struct held_lock	hlock;
+};
+
+struct lockdep_map_cross {
+	struct lockdep_map map;
+	struct cross_lock xlock;
+};
+#endif
+
 /*
  * Initialization, self-test and debugging-output methods:
  */
@@ -464,6 +557,37 @@  enum xhlock_context_t {
 	XHLOCK_CTX_NR,
 };
 
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+extern void lockdep_init_map_crosslock(struct lockdep_map *lock,
+				       const char *name,
+				       struct lock_class_key *key,
+				       int subclass);
+extern void lock_commit_crosslock(struct lockdep_map *lock);
+
+/*
+ * What we essencially have to initialize is 'nr_acquire'. Other members
+ * will be initialized in add_xlock().
+ */
+#define STATIC_CROSS_LOCK_INIT() \
+	{ .nr_acquire = 0,}
+
+#define STATIC_CROSS_LOCKDEP_MAP_INIT(_name, _key) \
+	{ .map.name = (_name), .map.key = (void *)(_key), \
+	  .map.cross = 1, .xlock = STATIC_CROSS_LOCK_INIT(), }
+
+/*
+ * To initialize a lockdep_map statically use this macro.
+ * Note that _name must not be NULL.
+ */
+#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
+	{ .name = (_name), .key = (void *)(_key), .cross = 0, }
+
+extern void crossrelease_hist_start(enum xhlock_context_t c);
+extern void crossrelease_hist_end(enum xhlock_context_t c);
+extern void lockdep_invariant_state(bool force);
+extern void lockdep_init_task(struct task_struct *task);
+extern void lockdep_free_task(struct task_struct *task);
+#else /* !CROSSRELEASE */
 #define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
 /*
  * To initialize a lockdep_map statically use this macro.
@@ -472,9 +596,12 @@  enum xhlock_context_t {
 #define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
 	{ .name = (_name), .key = (void *)(_key), }
 
+static inline void crossrelease_hist_start(enum xhlock_context_t c) {}
+static inline void crossrelease_hist_end(enum xhlock_context_t c) {}
 static inline void lockdep_invariant_state(bool force) {}
 static inline void lockdep_init_task(struct task_struct *task) {}
 static inline void lockdep_free_task(struct task_struct *task) {}
+#endif /* CROSSRELEASE */
 
 #ifdef CONFIG_LOCK_STAT
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57d7bc9..155c7690eb15 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -936,6 +936,17 @@  struct task_struct {
 	struct held_lock		held_locks[MAX_LOCK_DEPTH];
 #endif
 
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#define MAX_XHLOCKS_NR 64UL
+	struct hist_lock *xhlocks; /* Crossrelease history locks */
+	unsigned int xhlock_idx;
+	/* For restoring at history boundaries */
+	unsigned int xhlock_idx_hist[XHLOCK_CTX_NR];
+	unsigned int hist_id;
+	/* For overwrite check at each context exit */
+	unsigned int hist_id_save[XHLOCK_CTX_NR];
+#endif
+
 #ifdef CONFIG_UBSAN
 	unsigned int			in_ubsan;
 #endif
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index dd13f865ad40..c4d0fe3538da 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -58,6 +58,10 @@ 
 #define CREATE_TRACE_POINTS
 #include <trace/events/lock.h>
 
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#include <linux/slab.h>
+#endif
+
 #ifdef CONFIG_PROVE_LOCKING
 int prove_locking = 1;
 module_param(prove_locking, int, 0644);
@@ -72,6 +76,19 @@  module_param(lock_stat, int, 0644);
 #define lock_stat 0
 #endif
 
+#ifdef CONFIG_BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
+static int crossrelease_fullstack = 1;
+#else
+static int crossrelease_fullstack;
+#endif
+static int __init allow_crossrelease_fullstack(char *str)
+{
+	crossrelease_fullstack = 1;
+	return 0;
+}
+
+early_param("crossrelease_fullstack", allow_crossrelease_fullstack);
+
 /*
  * lockdep_lock: protects the lockdep graph, the hashes and the
  *               class/list/hash allocators.
@@ -731,6 +748,18 @@  static bool assign_lock_key(struct lockdep_map *lock)
 	return true;
 }
 
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+static void cross_init(struct lockdep_map *lock, int cross);
+static int cross_lock(struct lockdep_map *lock);
+static int lock_acquire_crosslock(struct held_lock *hlock);
+static int lock_release_crosslock(struct lockdep_map *lock);
+#else
+static inline void cross_init(struct lockdep_map *lock, int cross) {}
+static inline int cross_lock(struct lockdep_map *lock) { return 0; }
+static inline int lock_acquire_crosslock(struct held_lock *hlock) { return 2; }
+static inline int lock_release_crosslock(struct lockdep_map *lock) { return 2; }
+#endif
+
 /*
  * Register a lock's class in the hash-table, if the class is not present
  * yet. Otherwise we look it up. We cache the result in the lock object
@@ -1125,22 +1154,41 @@  print_circular_lock_scenario(struct held_lock *src,
 		printk(KERN_CONT "\n\n");
 	}
 
-	printk(" Possible unsafe locking scenario:\n\n");
-	printk("       CPU0                    CPU1\n");
-	printk("       ----                    ----\n");
-	printk("  lock(");
-	__print_lock_name(target);
-	printk(KERN_CONT ");\n");
-	printk("                               lock(");
-	__print_lock_name(parent);
-	printk(KERN_CONT ");\n");
-	printk("                               lock(");
-	__print_lock_name(target);
-	printk(KERN_CONT ");\n");
-	printk("  lock(");
-	__print_lock_name(source);
-	printk(KERN_CONT ");\n");
-	printk("\n *** DEADLOCK ***\n\n");
+	if (cross_lock(tgt->instance)) {
+		printk(" Possible unsafe locking scenario by crosslock:\n\n");
+		printk("       CPU0                    CPU1\n");
+		printk("       ----                    ----\n");
+		printk("  lock(");
+		__print_lock_name(parent);
+		printk(KERN_CONT ");\n");
+		printk("  lock(");
+		__print_lock_name(target);
+		printk(KERN_CONT ");\n");
+		printk("                               lock(");
+		__print_lock_name(source);
+		printk(KERN_CONT ");\n");
+		printk("                               unlock(");
+		__print_lock_name(target);
+		printk(KERN_CONT ");\n");
+		printk("\n *** DEADLOCK ***\n\n");
+	} else {
+		printk(" Possible unsafe locking scenario:\n\n");
+		printk("       CPU0                    CPU1\n");
+		printk("       ----                    ----\n");
+		printk("  lock(");
+		__print_lock_name(target);
+		printk(KERN_CONT ");\n");
+		printk("                               lock(");
+		__print_lock_name(parent);
+		printk(KERN_CONT ");\n");
+		printk("                               lock(");
+		__print_lock_name(target);
+		printk(KERN_CONT ");\n");
+		printk("  lock(");
+		__print_lock_name(source);
+		printk(KERN_CONT ");\n");
+		printk("\n *** DEADLOCK ***\n\n");
+	}
 }
 
 /*
@@ -1166,7 +1214,10 @@  print_circular_bug_header(struct lock_list *entry, unsigned int depth,
 		curr->comm, task_pid_nr(curr));
 	print_lock(check_src);
 
-	pr_warn("\nbut task is already holding lock:\n");
+	if (cross_lock(check_tgt->instance))
+		pr_warn("\nbut now in release context of a crosslock acquired at the following:\n");
+	else
+		pr_warn("\nbut task is already holding lock:\n");
 
 	print_lock(check_tgt);
 	pr_warn("\nwhich lock already depends on the new lock.\n\n");
@@ -1196,7 +1247,9 @@  static noinline int print_circular_bug(struct lock_list *this,
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
 		return 0;
 
-	if (!save_trace(&this->trace))
+	if (cross_lock(check_tgt->instance))
+		this->trace = *trace;
+	else if (!save_trace(&this->trace))
 		return 0;
 
 	depth = get_lock_depth(target);
@@ -1800,6 +1853,9 @@  check_deadlock(struct task_struct *curr, struct held_lock *next,
 		if (nest)
 			return 2;
 
+		if (cross_lock(prev->instance))
+			continue;
+
 		return print_deadlock_bug(curr, prev, next);
 	}
 	return 1;
@@ -1965,26 +2021,31 @@  check_prevs_add(struct task_struct *curr, struct held_lock *next)
 	for (;;) {
 		int distance = curr->lockdep_depth - depth + 1;
 		hlock = curr->held_locks + depth - 1;
-
 		/*
-		 * Only non-recursive-read entries get new dependencies
-		 * added:
+		 * Only non-crosslock entries get new dependencies added.
+		 * Crosslock entries will be added by commit later:
 		 */
-		if (hlock->read != 2 && hlock->check) {
-			int ret = check_prev_add(curr, hlock, next, distance, &trace, save_trace);
-			if (!ret)
-				return 0;
-
+		if (!cross_lock(hlock->instance)) {
 			/*
-			 * Stop after the first non-trylock entry,
-			 * as non-trylock entries have added their
-			 * own direct dependencies already, so this
-			 * lock is connected to them indirectly:
+			 * Only non-recursive-read entries get new dependencies
+			 * added:
 			 */
-			if (!hlock->trylock)
-				break;
-		}
+			if (hlock->read != 2 && hlock->check) {
+				int ret = check_prev_add(curr, hlock, next,
+							 distance, &trace, save_trace);
+				if (!ret)
+					return 0;
 
+				/*
+				 * Stop after the first non-trylock entry,
+				 * as non-trylock entries have added their
+				 * own direct dependencies already, so this
+				 * lock is connected to them indirectly:
+				 */
+				if (!hlock->trylock)
+					break;
+			}
+		}
 		depth--;
 		/*
 		 * End of lock-stack?
@@ -3216,10 +3277,21 @@  static void __lockdep_init_map(struct lockdep_map *lock, const char *name,
 void lockdep_init_map(struct lockdep_map *lock, const char *name,
 		      struct lock_class_key *key, int subclass)
 {
+	cross_init(lock, 0);
 	__lockdep_init_map(lock, name, key, subclass);
 }
 EXPORT_SYMBOL_GPL(lockdep_init_map);
 
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+void lockdep_init_map_crosslock(struct lockdep_map *lock, const char *name,
+		      struct lock_class_key *key, int subclass)
+{
+	cross_init(lock, 1);
+	__lockdep_init_map(lock, name, key, subclass);
+}
+EXPORT_SYMBOL_GPL(lockdep_init_map_crosslock);
+#endif
+
 struct lock_class_key __lockdep_no_validate__;
 EXPORT_SYMBOL_GPL(__lockdep_no_validate__);
 
@@ -3275,6 +3347,7 @@  static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	int chain_head = 0;
 	int class_idx;
 	u64 chain_key;
+	int ret;
 
 	if (unlikely(!debug_locks))
 		return 0;
@@ -3323,7 +3396,8 @@  static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	class_idx = class - lock_classes + 1;
 
-	if (depth) {
+	/* TODO: nest_lock is not implemented for crosslock yet. */
+	if (depth && !cross_lock(lock)) {
 		hlock = curr->held_locks + depth - 1;
 		if (hlock->class_idx == class_idx && nest_lock) {
 			if (hlock->references) {
@@ -3411,6 +3485,14 @@  static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
 		return 0;
 
+	ret = lock_acquire_crosslock(hlock);
+	/*
+	 * 2 means normal acquire operations are needed. Otherwise, it's
+	 * ok just to return with '0:fail, 1:success'.
+	 */
+	if (ret != 2)
+		return ret;
+
 	curr->curr_chain_key = chain_key;
 	curr->lockdep_depth++;
 	check_chain_key(curr);
@@ -3649,11 +3731,19 @@  __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
 	struct task_struct *curr = current;
 	struct held_lock *hlock;
 	unsigned int depth;
-	int i;
+	int ret, i;
 
 	if (unlikely(!debug_locks))
 		return 0;
 
+	ret = lock_release_crosslock(lock);
+	/*
+	 * 2 means normal release operations are needed. Otherwise, it's
+	 * ok just to return with '0:fail, 1:success'.
+	 */
+	if (ret != 2)
+		return ret;
+
 	depth = curr->lockdep_depth;
 	/*
 	 * So we're all set to release this lock.. wait what lock? We don't
@@ -4536,3 +4626,495 @@  void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
 	dump_stack();
 }
 EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
+
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+
+/*
+ * Crossrelease works by recording a lock history for each thread and
+ * connecting those historic locks that were taken after the
+ * wait_for_completion() in the complete() context.
+ *
+ * Task-A				Task-B
+ *
+ *					mutex_lock(&A);
+ *					mutex_unlock(&A);
+ *
+ * wait_for_completion(&C);
+ *   lock_acquire_crosslock();
+ *     atomic_inc_return(&cross_gen_id);
+ *                                |
+ *				  |	mutex_lock(&B);
+ *				  |	mutex_unlock(&B);
+ *                                |
+ *				  |	complete(&C);
+ *				  `--	  lock_commit_crosslock();
+ *
+ * Which will then add a dependency between B and C.
+ */
+
+#define xhlock(i)         (current->xhlocks[(i) % MAX_XHLOCKS_NR])
+
+/*
+ * Whenever a crosslock is held, cross_gen_id will be increased.
+ */
+static atomic_t cross_gen_id; /* Can be wrapped */
+
+/*
+ * Make an entry of the ring buffer invalid.
+ */
+static inline void invalidate_xhlock(struct hist_lock *xhlock)
+{
+	/*
+	 * Normally, xhlock->hlock.instance must be !NULL.
+	 */
+	xhlock->hlock.instance = NULL;
+}
+
+/*
+ * Lock history stacks; we have 2 nested lock history stacks:
+ *
+ *   HARD(IRQ)
+ *   SOFT(IRQ)
+ *
+ * The thing is that once we complete a HARD/SOFT IRQ the future task locks
+ * should not depend on any of the locks observed while running the IRQ.  So
+ * what we do is rewind the history buffer and erase all our knowledge of that
+ * temporal event.
+ */
+
+void crossrelease_hist_start(enum xhlock_context_t c)
+{
+	struct task_struct *cur = current;
+
+	if (!cur->xhlocks)
+		return;
+
+	cur->xhlock_idx_hist[c] = cur->xhlock_idx;
+	cur->hist_id_save[c]    = cur->hist_id;
+}
+
+void crossrelease_hist_end(enum xhlock_context_t c)
+{
+	struct task_struct *cur = current;
+
+	if (cur->xhlocks) {
+		unsigned int idx = cur->xhlock_idx_hist[c];
+		struct hist_lock *h = &xhlock(idx);
+
+		cur->xhlock_idx = idx;
+
+		/* Check if the ring was overwritten. */
+		if (h->hist_id != cur->hist_id_save[c])
+			invalidate_xhlock(h);
+	}
+}
+
+/*
+ * lockdep_invariant_state() is used to annotate independence inside a task, to
+ * make one task look like multiple independent 'tasks'.
+ *
+ * Take for instance workqueues; each work is independent of the last. The
+ * completion of a future work does not depend on the completion of a past work
+ * (in general). Therefore we must not carry that (lock) dependency across
+ * works.
+ *
+ * This is true for many things; pretty much all kthreads fall into this
+ * pattern, where they have an invariant state and future completions do not
+ * depend on past completions. Its just that since they all have the 'same'
+ * form -- the kthread does the same over and over -- it doesn't typically
+ * matter.
+ *
+ * The same is true for system-calls, once a system call is completed (we've
+ * returned to userspace) the next system call does not depend on the lock
+ * history of the previous system call.
+ *
+ * They key property for independence, this invariant state, is that it must be
+ * a point where we hold no locks and have no history. Because if we were to
+ * hold locks, the restore at _end() would not necessarily recover it's history
+ * entry. Similarly, independence per-definition means it does not depend on
+ * prior state.
+ */
+void lockdep_invariant_state(bool force)
+{
+	/*
+	 * We call this at an invariant point, no current state, no history.
+	 * Verify the former, enforce the latter.
+	 */
+	WARN_ON_ONCE(!force && current->lockdep_depth);
+	if (current->xhlocks)
+		invalidate_xhlock(&xhlock(current->xhlock_idx));
+}
+
+static int cross_lock(struct lockdep_map *lock)
+{
+	return lock ? lock->cross : 0;
+}
+
+/*
+ * This is needed to decide the relationship between wrapable variables.
+ */
+static inline int before(unsigned int a, unsigned int b)
+{
+	return (int)(a - b) < 0;
+}
+
+static inline struct lock_class *xhlock_class(struct hist_lock *xhlock)
+{
+	return hlock_class(&xhlock->hlock);
+}
+
+static inline struct lock_class *xlock_class(struct cross_lock *xlock)
+{
+	return hlock_class(&xlock->hlock);
+}
+
+/*
+ * Should we check a dependency with previous one?
+ */
+static inline int depend_before(struct held_lock *hlock)
+{
+	return hlock->read != 2 && hlock->check && !hlock->trylock;
+}
+
+/*
+ * Should we check a dependency with next one?
+ */
+static inline int depend_after(struct held_lock *hlock)
+{
+	return hlock->read != 2 && hlock->check;
+}
+
+/*
+ * Check if the xhlock is valid, which would be false if,
+ *
+ *    1. Has not used after initializaion yet.
+ *    2. Got invalidated.
+ *
+ * Remind hist_lock is implemented as a ring buffer.
+ */
+static inline int xhlock_valid(struct hist_lock *xhlock)
+{
+	/*
+	 * xhlock->hlock.instance must be !NULL.
+	 */
+	return !!xhlock->hlock.instance;
+}
+
+/*
+ * Record a hist_lock entry.
+ *
+ * Irq disable is only required.
+ */
+static void add_xhlock(struct held_lock *hlock)
+{
+	unsigned int idx = ++current->xhlock_idx;
+	struct hist_lock *xhlock = &xhlock(idx);
+
+#ifdef CONFIG_DEBUG_LOCKDEP
+	/*
+	 * This can be done locklessly because they are all task-local
+	 * state, we must however ensure IRQs are disabled.
+	 */
+	WARN_ON_ONCE(!irqs_disabled());
+#endif
+
+	/* Initialize hist_lock's members */
+	xhlock->hlock = *hlock;
+	xhlock->hist_id = ++current->hist_id;
+
+	xhlock->trace.nr_entries = 0;
+	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
+	xhlock->trace.entries = xhlock->trace_entries;
+
+	if (crossrelease_fullstack) {
+		xhlock->trace.skip = 3;
+		save_stack_trace(&xhlock->trace);
+	} else {
+		xhlock->trace.nr_entries = 1;
+		xhlock->trace.entries[0] = hlock->acquire_ip;
+	}
+}
+
+static inline int same_context_xhlock(struct hist_lock *xhlock)
+{
+	return xhlock->hlock.irq_context == task_irq_context(current);
+}
+
+/*
+ * This should be lockless as far as possible because this would be
+ * called very frequently.
+ */
+static void check_add_xhlock(struct held_lock *hlock)
+{
+	/*
+	 * Record a hist_lock, only in case that acquisitions ahead
+	 * could depend on the held_lock. For example, if the held_lock
+	 * is trylock then acquisitions ahead never depends on that.
+	 * In that case, we don't need to record it. Just return.
+	 */
+	if (!current->xhlocks || !depend_before(hlock))
+		return;
+
+	add_xhlock(hlock);
+}
+
+/*
+ * For crosslock.
+ */
+static int add_xlock(struct held_lock *hlock)
+{
+	struct cross_lock *xlock;
+	unsigned int gen_id;
+
+	if (!graph_lock())
+		return 0;
+
+	xlock = &((struct lockdep_map_cross *)hlock->instance)->xlock;
+
+	/*
+	 * When acquisitions for a crosslock are overlapped, we use
+	 * nr_acquire to perform commit for them, based on cross_gen_id
+	 * of the first acquisition, which allows to add additional
+	 * dependencies.
+	 *
+	 * Moreover, when no acquisition of a crosslock is in progress,
+	 * we should not perform commit because the lock might not exist
+	 * any more, which might cause incorrect memory access. So we
+	 * have to track the number of acquisitions of a crosslock.
+	 *
+	 * depend_after() is necessary to initialize only the first
+	 * valid xlock so that the xlock can be used on its commit.
+	 */
+	if (xlock->nr_acquire++ && depend_after(&xlock->hlock))
+		goto unlock;
+
+	gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
+	xlock->hlock = *hlock;
+	xlock->hlock.gen_id = gen_id;
+unlock:
+	graph_unlock();
+	return 1;
+}
+
+/*
+ * Called for both normal and crosslock acquires. Normal locks will be
+ * pushed on the hist_lock queue. Cross locks will record state and
+ * stop regular lock_acquire() to avoid being placed on the held_lock
+ * stack.
+ *
+ * Return: 0 - failure;
+ *         1 - crosslock, done;
+ *         2 - normal lock, continue to held_lock[] ops.
+ */
+static int lock_acquire_crosslock(struct held_lock *hlock)
+{
+	/*
+	 *	CONTEXT 1		CONTEXT 2
+	 *	---------		---------
+	 *	lock A (cross)
+	 *	X = atomic_inc_return(&cross_gen_id)
+	 *	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+	 *				Y = atomic_read_acquire(&cross_gen_id)
+	 *				lock B
+	 *
+	 * atomic_read_acquire() is for ordering between A and B,
+	 * IOW, A happens before B, when CONTEXT 2 see Y >= X.
+	 *
+	 * Pairs with atomic_inc_return() in add_xlock().
+	 */
+	hlock->gen_id = (unsigned int)atomic_read_acquire(&cross_gen_id);
+
+	if (cross_lock(hlock->instance))
+		return add_xlock(hlock);
+
+	check_add_xhlock(hlock);
+	return 2;
+}
+
+static int copy_trace(struct stack_trace *trace)
+{
+	unsigned long *buf = stack_trace + nr_stack_trace_entries;
+	unsigned int max_nr = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
+	unsigned int nr = min(max_nr, trace->nr_entries);
+
+	trace->nr_entries = nr;
+	memcpy(buf, trace->entries, nr * sizeof(trace->entries[0]));
+	trace->entries = buf;
+	nr_stack_trace_entries += nr;
+
+	if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES-1) {
+		if (!debug_locks_off_graph_unlock())
+			return 0;
+
+		print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
+		dump_stack();
+
+		return 0;
+	}
+
+	return 1;
+}
+
+static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
+{
+	unsigned int xid, pid;
+	u64 chain_key;
+
+	xid = xlock_class(xlock) - lock_classes;
+	chain_key = iterate_chain_key((u64)0, xid);
+	pid = xhlock_class(xhlock) - lock_classes;
+	chain_key = iterate_chain_key(chain_key, pid);
+
+	if (lookup_chain_cache(chain_key))
+		return 1;
+
+	if (!add_chain_cache_classes(xid, pid, xhlock->hlock.irq_context,
+				chain_key))
+		return 0;
+
+	if (!check_prev_add(current, &xlock->hlock, &xhlock->hlock, 1,
+			    &xhlock->trace, copy_trace))
+		return 0;
+
+	return 1;
+}
+
+static void commit_xhlocks(struct cross_lock *xlock)
+{
+	unsigned int cur = current->xhlock_idx;
+	unsigned int prev_hist_id = xhlock(cur).hist_id;
+	unsigned int i;
+
+	if (!graph_lock())
+		return;
+
+	if (xlock->nr_acquire) {
+		for (i = 0; i < MAX_XHLOCKS_NR; i++) {
+			struct hist_lock *xhlock = &xhlock(cur - i);
+
+			if (!xhlock_valid(xhlock))
+				break;
+
+			if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
+				break;
+
+			if (!same_context_xhlock(xhlock))
+				break;
+
+			/*
+			 * Filter out the cases where the ring buffer was
+			 * overwritten and the current entry has a bigger
+			 * hist_id than the previous one, which is impossible
+			 * otherwise:
+			 */
+			if (unlikely(before(prev_hist_id, xhlock->hist_id)))
+				break;
+
+			prev_hist_id = xhlock->hist_id;
+
+			/*
+			 * commit_xhlock() returns 0 with graph_lock already
+			 * released if fail.
+			 */
+			if (!commit_xhlock(xlock, xhlock))
+				return;
+		}
+	}
+
+	graph_unlock();
+}
+
+void lock_commit_crosslock(struct lockdep_map *lock)
+{
+	struct cross_lock *xlock;
+	unsigned long flags;
+
+	if (unlikely(!debug_locks || current->lockdep_recursion))
+		return;
+
+	if (!current->xhlocks)
+		return;
+
+	/*
+	 * Do commit hist_locks with the cross_lock, only in case that
+	 * the cross_lock could depend on acquisitions after that.
+	 *
+	 * For example, if the cross_lock does not have the 'check' flag
+	 * then we don't need to check dependencies and commit for that.
+	 * Just skip it. In that case, of course, the cross_lock does
+	 * not depend on acquisitions ahead, either.
+	 *
+	 * WARNING: Don't do that in add_xlock() in advance. When an
+	 * acquisition context is different from the commit context,
+	 * invalid(skipped) cross_lock might be accessed.
+	 */
+	if (!depend_after(&((struct lockdep_map_cross *)lock)->xlock.hlock))
+		return;
+
+	raw_local_irq_save(flags);
+	check_flags(flags);
+	current->lockdep_recursion = 1;
+	xlock = &((struct lockdep_map_cross *)lock)->xlock;
+	commit_xhlocks(xlock);
+	current->lockdep_recursion = 0;
+	raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_commit_crosslock);
+
+/*
+ * Return: 0 - failure;
+ *         1 - crosslock, done;
+ *         2 - normal lock, continue to held_lock[] ops.
+ */
+static int lock_release_crosslock(struct lockdep_map *lock)
+{
+	if (cross_lock(lock)) {
+		if (!graph_lock())
+			return 0;
+		((struct lockdep_map_cross *)lock)->xlock.nr_acquire--;
+		graph_unlock();
+		return 1;
+	}
+	return 2;
+}
+
+static void cross_init(struct lockdep_map *lock, int cross)
+{
+	if (cross)
+		((struct lockdep_map_cross *)lock)->xlock.nr_acquire = 0;
+
+	lock->cross = cross;
+
+	/*
+	 * Crossrelease assumes that the ring buffer size of xhlocks
+	 * is aligned with power of 2. So force it on build.
+	 */
+	BUILD_BUG_ON(MAX_XHLOCKS_NR & (MAX_XHLOCKS_NR - 1));
+}
+
+void lockdep_init_task(struct task_struct *task)
+{
+	int i;
+
+	task->xhlock_idx = UINT_MAX;
+	task->hist_id = 0;
+
+	for (i = 0; i < XHLOCK_CTX_NR; i++) {
+		task->xhlock_idx_hist[i] = UINT_MAX;
+		task->hist_id_save[i] = 0;
+	}
+
+	task->xhlocks = kzalloc(sizeof(struct hist_lock) * MAX_XHLOCKS_NR,
+				GFP_KERNEL);
+}
+
+void lockdep_free_task(struct task_struct *task)
+{
+	if (task->xhlocks) {
+		void *tmp = task->xhlocks;
+		/* Diable crossrelease for current */
+		task->xhlocks = NULL;
+		kfree(tmp);
+	}
+}
+#endif
diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index a1ad5b7d5521..0f7c442f9dd7 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -31,6 +31,11 @@  void complete(struct completion *x)
 
 	spin_lock_irqsave(&x->wait.lock, flags);
 
+	/*
+	 * Perform commit of crossrelease here.
+	 */
+	complete_release_commit(x);
+
 	if (x->done != UINT_MAX)
 		x->done++;
 	__wake_up_locked(&x->wait, TASK_NORMAL, 1);
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1ead06829fdb..fbfdf7c263d2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1055,6 +1055,8 @@  config PROVE_LOCKING
 	select DEBUG_RWSEMS if RWSEM_SPIN_ON_OWNER
 	select DEBUG_WW_MUTEX_SLOWPATH
 	select DEBUG_LOCK_ALLOC
+	select LOCKDEP_CROSSRELEASE
+	select LOCKDEP_COMPLETIONS
 	select TRACE_IRQFLAGS
 	default n
 	help
@@ -1187,6 +1189,37 @@  config LOCKDEP
 config LOCKDEP_SMALL
 	bool
 
+config LOCKDEP_CROSSRELEASE
+	bool
+	help
+	 This makes lockdep work for crosslock which is a lock allowed to
+	 be released in a different context from the acquisition context.
+	 Normally a lock must be released in the context acquiring the lock.
+	 However, relexing this constraint helps synchronization primitives
+	 such as page locks or completions can use the lock correctness
+	 detector, lockdep.
+
+config LOCKDEP_COMPLETIONS
+	bool
+	help
+	 A deadlock caused by wait_for_completion() and complete() can be
+	 detected by lockdep using crossrelease feature.
+
+config BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
+	bool "Enable the boot parameter, crossrelease_fullstack"
+	depends on LOCKDEP_CROSSRELEASE
+	default n
+	help
+	 The lockdep "cross-release" feature needs to record stack traces
+	 (of calling functions) for all acquisitions, for eventual later
+	 use during analysis. By default only a single caller is recorded,
+	 because the unwind operation can be very expensive with deeper
+	 stack chains.
+
+	 However a boot parameter, crossrelease_fullstack, was
+	 introduced since sometimes deeper traces are required for full
+	 analysis. This option turns on the boot parameter.
+
 config DEBUG_LOCKDEP
 	bool "Lock dependency engine debugging"
 	depends on DEBUG_KERNEL && LOCKDEP