From: Dan Williams dan.j.williams@intel.com Subject: mm/memory_hotplug: fix remove_memory() lockdep splat

The daxctl unit test for the dax_kmem driver currently triggers the (false positive) 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() and sysfs (kernfs active state tracking). It is a false positive because the sysfs attribute path triggering the memory remove is not the same attribute path associated with memory-block device.

sysfs_break_active_protection() is not applicable since there is no real deadlock conflict, instead move memory-block device removal outside the lock. The mem_hotplug_lock() is not needed to synchronize the memory-block device removal 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 any in progress online attempts are flushed / blocked by kernfs_drain() / attribute removal.

The add_memory() path safely creates memblock devices under the mem_hotplug_lock(). There is no kernfs active state synchronization in the memblock device_register() path, so nothing to fix there.

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()), and David's due diligence tracking down the guarantees afforded by kernfs_drain(). Not flagged for -stable since this only impacts ongoing development and lockdep validation, not a runtime issue.

====================================================== 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 has been a long standing issue that predated the addition of kernfs lockdep annotations.

Link: http://lkml.kernel.org/r/157991441887.2763922.4770790047389427325.stgit@dwil... Signed-off-by: Dan Williams dan.j.williams@intel.com Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: David Hildenbrand david@redhat.com Cc: Vishal Verma vishal.l.verma@intel.com Cc: Pavel Tatashin pasha.tatashin@soleen.com Cc: Dave Hansen dave.hansen@linux.intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memory_hotplug.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/mm/memory_hotplug.c~mm-memory_hotplug-fix-remove_memory-lockdep-splat +++ a/mm/memory_hotplug.c @@ -1764,8 +1764,6 @@ static int __ref try_remove_memory(int n

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 @@ -1778,9 +1776,14 @@ static int __ref try_remove_memory(int n /* remove memmap entry */ firmware_map_remove(start, start + size, "System RAM");

- /* remove memory block devices before removing memory */ + /* + * Memory block device removal under the device_hotplug_lock is + * a barrier against racing online attempts. + */ remove_memory_block_devices(start, size);

+ mem_hotplug_begin(); + arch_remove_memory(nid, start, size, NULL); memblock_free(start, size); memblock_remove(start, size); _