@@ -90,12 +90,13 @@ Locking Internals
=================
When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
-the device_hotplug_lock should be held to:
+the device_hotplug_lock is held to:
- synchronize against online/offline requests (e.g. via sysfs). This way, memory
block devices can only be accessed (.online/.state attributes) by user
- space once memory has been fully added. And when removing memory, we
- know nobody is in critical sections.
+ space once memory has been fully added. And when removing memory, the
+ memory block device is invalidated (mem->section count set to 0) under the
+ lock to abort any in-flight online requests.
- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
Especially, there is a possible lock inversion that is avoided using
@@ -112,7 +113,13 @@ can result in a lock inversion.
onlining/offlining of memory should be done via device_online()/
device_offline() - to make sure it is properly synchronized to actions
-via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
+via sysfs. Holding device_hotplug_lock is required to prevent online racing
+removal. The device_hotplug_lock and memblock invalidation allows
+remove_memory_block_devices() to run outside of mem_hotplug_lock to avoid lock
+dependency conflicts with memblock-sysfs teardown. The add_memory() path
+performs create_memory_block_devices() under mem_hotplug_lock so that if it
+fails it can perform an arch_remove_memory() cleanup. There are no known lock
+dependency problems with memblock-sysfs setup.
When adding/removing/onlining/offlining memory or adding/removing
heterogeneous/device memory, we should always hold the mem_hotplug_lock in
@@ -1146,6 +1146,11 @@ void unlock_device_hotplug(void)
mutex_unlock(&device_hotplug_lock);
}
+void assert_held_device_hotplug(void)
+{
+ lockdep_assert_held(&device_hotplug_lock);
+}
+
int lock_device_hotplug_sysfs(void)
{
if (mutex_trylock(&device_hotplug_lock))
@@ -280,6 +280,10 @@ static int memory_subsys_online(struct device *dev)
if (mem->state == MEM_ONLINE)
return 0;
+ /* online lost the race with hot-unplug, abort */
+ if (!mem->section_count)
+ return -ENXIO;
+
/*
* If we are called from state_store(), online_type will be
* set >= 0 Otherwise we were called from the device online
@@ -736,8 +740,6 @@ int create_memory_block_devices(unsigned long start, unsigned long size)
* Remove memory block devices for the given memory area. Start and size
* have to be aligned to memory block granularity. Memory block devices
* have to be offline.
- *
- * Called under device_hotplug_lock.
*/
void remove_memory_block_devices(unsigned long start, unsigned long size)
{
@@ -746,6 +748,8 @@ void remove_memory_block_devices(unsigned long start, unsigned long size)
struct memory_block *mem;
unsigned long block_id;
+ assert_held_device_hotplug();
+
if (WARN_ON_ONCE(!IS_ALIGNED(start, memory_block_size_bytes()) ||
!IS_ALIGNED(size, memory_block_size_bytes())))
return;
@@ -1553,6 +1553,7 @@ static inline bool device_supports_offline(struct device *dev)
extern void lock_device_hotplug(void);
extern void unlock_device_hotplug(void);
extern int lock_device_hotplug_sysfs(void);
+extern void assert_held_device_hotlpug(void);
extern int device_offline(struct device *dev);
extern int device_online(struct device *dev);
extern void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode);
@@ -1763,8 +1763,6 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
BUG_ON(check_hotplug_memory_range(start, size));
- mem_hotplug_begin();
-
/*
* All memory blocks must be offlined before removing memory. Check
* whether all memory blocks in question are offline and return error
@@ -1777,9 +1775,15 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
/* remove memmap entry */
firmware_map_remove(start, start + size, "System RAM");
- /* remove memory block devices before removing memory */
+ /*
+ * Remove memory block devices before removing memory and before
+ * mem_hotplug_begin() (see Documentation/core-api/memory-hotplug.rst
+ * "Locking Internals").
+ */
remove_memory_block_devices(start, size);
+ mem_hotplug_begin();
+
arch_remove_memory(nid, start, size, NULL);
memblock_free(start, size);
memblock_remove(start, size);
The daxctl unit test for the dax_kmem driver currently triggers the lockdep splat below. It results from the fact that remove_memory_block_devices() is invoked under the mem_hotplug_lock() causing lockdep entanglements with cpu_hotplug_lock(). The mem_hotplug_lock() is not needed to synchronize the memory block device sysfs interface vs the page online state, that is already handled by lock_device_hotplug(). Specifically lock_device_hotplug() is sufficient to allow try_remove_memory() to check the offline state of the memblocks and be assured that subsequent online attempts will be blocked. The device_online() path checks mem->section_count before allowing any state manipulations and mem->section_count is cleared in remove_memory_block_devices(). The add_memory() path does create memblock devices under the lock, but there is no lockdep report on that path, and it wants to unwind the hot-add (via arch_remove_memory()) if the memblock device creation fails, so it is left alone for now. This change is only possible thanks to the recent change that refactored memory block device removal out of arch_remove_memory() (commit 4c4b7f9ba948 mm/memory_hotplug: remove memory block devices before arch_remove_memory()). ====================================================== WARNING: possible circular locking dependency detected 5.5.0-rc3+ #230 Tainted: G OE ------------------------------------------------------ lt-daxctl/6459 is trying to acquire lock: ffff99c7f0003510 (kn->count#241){++++}, at: kernfs_remove_by_name_ns+0x41/0x80 but task is already holding lock: ffffffffa76a5450 (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x20/0xe0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (mem_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 get_online_mems+0x3e/0xb0 kmem_cache_create_usercopy+0x2e/0x260 kmem_cache_create+0x12/0x20 ptlock_cache_init+0x20/0x28 start_kernel+0x243/0x547 secondary_startup_64+0xb6/0xc0 -> #1 (cpu_hotplug_lock.rw_sem){++++}: __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 cpus_read_lock+0x3e/0xb0 online_pages+0x37/0x300 memory_subsys_online+0x17d/0x1c0 device_online+0x60/0x80 state_store+0x65/0xd0 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#241){++++}: check_prev_add+0x98/0xa40 validate_chain+0x576/0x860 __lock_acquire+0x39c/0x790 lock_acquire+0xa2/0x1b0 __kernfs_remove+0x25f/0x2e0 kernfs_remove_by_name_ns+0x41/0x80 remove_files.isra.0+0x30/0x70 sysfs_remove_group+0x3d/0x80 sysfs_remove_groups+0x29/0x40 device_remove_attrs+0x39/0x70 device_del+0x16a/0x3f0 device_unregister+0x16/0x60 remove_memory_block_devices+0x82/0xb0 try_remove_memory+0xb5/0x130 remove_memory+0x26/0x40 dev_dax_kmem_remove+0x44/0x6a [kmem] device_release_driver_internal+0xe4/0x1c0 unbind_store+0xef/0x120 kernfs_fop_write+0xcf/0x1c0 vfs_write+0xdb/0x1d0 ksys_write+0x65/0xe0 do_syscall_64+0x5c/0xa0 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Chain exists of: kn->count#241 --> cpu_hotplug_lock.rw_sem --> mem_hotplug_lock.rw_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(mem_hotplug_lock.rw_sem); lock(cpu_hotplug_lock.rw_sem); lock(mem_hotplug_lock.rw_sem); lock(kn->count#241); *** DEADLOCK *** No fixes tag as this seems to have been a long standing issue that likely predated the addition of kernfs lockdep annotations. Cc: <stable@vger.kernel.org> Cc: Vishal Verma <vishal.l.verma@intel.com> Cc: David Hildenbrand <david@redhat.com> Cc: Pavel Tatashin <pasha.tatashin@soleen.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> --- Changes since v2 [1]: - Apologies I overlooked that I had local changes in my tree to fix a compiler error (misspelled assert_held_device_hotplug()). Now fixed up. [1]: http://lore.kernel.org/r/157868867304.2306270.4899678179641333013.stgit@dwillia2-desk3.amr.corp.intel.com Documentation/core-api/memory-hotplug.rst | 15 +++++++++++---- drivers/base/core.c | 5 +++++ drivers/base/memory.c | 8 ++++++-- include/linux/device.h | 1 + mm/memory_hotplug.c | 10 +++++++--- 5 files changed, 30 insertions(+), 9 deletions(-)