diff mbox series

[RFC,1/4] locking/percpu-rwsem: add freezable alternative to down_read

Message ID 20250327140613.25178-2-James.Bottomley@HansenPartnership.com (mailing list archive)
State New
Headers show
Series vfs freeze/thaw on suspend/resume | expand

Commit Message

James Bottomley March 27, 2025, 2:06 p.m. UTC 
Percpu-rwsems are used for superblock locking.  However, we know the
read percpu-rwsem we take for sb_start_write() on a frozen filesystem
needs not to inhibit system from suspending or hibernating.  That
means it needs to wait with TASK_UNINTERRUPTIBLE | TASK_FREEZABLE.

Introduce a new percpu_down_read_freezable() that allows us to control
whether TASK_FREEZABLE is added to the wait flags.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

---

Since this is an RFC, added the percpu-rwsem maintainers for
information and guidance to check if we're on the right track or
whether they would prefer an alternative API.
---
 include/linux/percpu-rwsem.h  | 20 ++++++++++++++++----
 kernel/locking/percpu-rwsem.c | 13 ++++++++-----
 2 files changed, 24 insertions(+), 9 deletions(-)

Comments

James Bottomley March 31, 2025, 7:51 p.m. UTC | #1 
On Thu, 2025-03-27 at 10:06 -0400, James Bottomley wrote:
[...]
> -static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool
> reader)
> +static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool
> reader,
> +			      bool freeze)
>  {
>  	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
>  	bool wait;
> @@ -156,7 +157,8 @@ static void percpu_rwsem_wait(struct
> percpu_rw_semaphore *sem, bool reader)
>  	spin_unlock_irq(&sem->waiters.lock);
>  
>  	while (wait) {
> -		set_current_state(TASK_UNINTERRUPTIBLE);
> +		set_current_state(TASK_UNINTERRUPTIBLE |
> +				  freeze ? TASK_FREEZABLE : 0);

This is a bit embarrassing, the bug I've been chasing is here: the ?
operator is lower in precedence than | meaning this expression always
evaluates to TASK_FREEZABLE and nothing else (which is why the process
goes into R state and never wakes up).

Let me fix that and redo all the testing.

Regards,

James
diff mbox series

Patch

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index c012df33a9f0..a55fe709b832 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -42,9 +42,10 @@  is_static struct percpu_rw_semaphore name = {				\
 #define DEFINE_STATIC_PERCPU_RWSEM(name)	\
 	__DEFINE_PERCPU_RWSEM(name, static)
 
-extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
+extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool, bool);
 
-static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+static inline void percpu_down_read_internal(struct percpu_rw_semaphore *sem,
+					     bool freezable)
 {
 	might_sleep();
 
@@ -62,7 +63,7 @@  static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
 	if (likely(rcu_sync_is_idle(&sem->rss)))
 		this_cpu_inc(*sem->read_count);
 	else
-		__percpu_down_read(sem, false); /* Unconditional memory barrier */
+		__percpu_down_read(sem, false, freezable); /* Unconditional memory barrier */
 	/*
 	 * The preempt_enable() prevents the compiler from
 	 * bleeding the critical section out.
@@ -70,6 +71,17 @@  static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
 	preempt_enable();
 }
 
+static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+{
+	percpu_down_read_internal(sem, false);
+}
+
+static inline void percpu_down_read_freezable(struct percpu_rw_semaphore *sem,
+					      bool freeze)
+{
+	percpu_down_read_internal(sem, freeze);
+}
+
 static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 {
 	bool ret = true;
@@ -81,7 +93,7 @@  static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 	if (likely(rcu_sync_is_idle(&sem->rss)))
 		this_cpu_inc(*sem->read_count);
 	else
-		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
+		ret = __percpu_down_read(sem, true, false); /* Unconditional memory barrier */
 	preempt_enable();
 	/*
 	 * The barrier() from preempt_enable() prevents the compiler from
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 6083883c4fe0..890837b73476 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -138,7 +138,8 @@  static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
 	return !reader; /* wake (readers until) 1 writer */
 }
 
-static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
+static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader,
+			      bool freeze)
 {
 	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
 	bool wait;
@@ -156,7 +157,8 @@  static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
 	spin_unlock_irq(&sem->waiters.lock);
 
 	while (wait) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE |
+				  freeze ? TASK_FREEZABLE : 0);
 		if (!smp_load_acquire(&wq_entry.private))
 			break;
 		schedule();
@@ -164,7 +166,8 @@  static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
 	__set_current_state(TASK_RUNNING);
 }
 
-bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
+bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try,
+				bool freeze)
 {
 	if (__percpu_down_read_trylock(sem))
 		return true;
@@ -174,7 +177,7 @@  bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
 
 	trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
 	preempt_enable();
-	percpu_rwsem_wait(sem, /* .reader = */ true);
+	percpu_rwsem_wait(sem, /* .reader = */ true, freeze);
 	preempt_disable();
 	trace_contention_end(sem, 0);
 
@@ -237,7 +240,7 @@  void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
 	 */
 	if (!__percpu_down_write_trylock(sem)) {
 		trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
-		percpu_rwsem_wait(sem, /* .reader = */ false);
+		percpu_rwsem_wait(sem, /* .reader = */ false, false);
 		contended = true;
 	}