Re: [PATCH v6] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim

[Waiman Long <longman@redhat.com>]

On 7/20/20 11:40 AM, Darrick J. Wong wrote:
> On Mon, Jul 20, 2020 at 11:32:03AM -0400, Waiman Long wrote:
>>
>> ----- Original Message -----
>> From: "Darrick J. Wong" <darrick.wong@oracle.com>
>> To: "Waiman Long" <longman@redhat.com>
>> Cc: linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, "Dave Chinner" <david@fromorbit.com>, "Qian Cai" <cai@lca.pw>, "Eric Sandeen" <sandeen@redhat.com>
>> Sent: Monday, July 13, 2020 12:41:12 PM
>> Subject: Re: [PATCH v6] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim
>>
>> On Tue, Jul 07, 2020 at 03:16:29PM -0400, Waiman Long wrote:
>>> Depending on the workloads, the following circular locking dependency
>>> warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo
>>> lock) may show up:
>>>
>>> ======================================================
>>> WARNING: possible circular locking dependency detected
>>> 5.0.0-rc1+ #60 Tainted: G        W
>>> ------------------------------------------------------
>>> fsfreeze/4346 is trying to acquire lock:
>>> 0000000026f1d784 (fs_reclaim){+.+.}, at:
>>> fs_reclaim_acquire.part.19+0x5/0x30
>>>
>>> but task is already holding lock:
>>> 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>>
>>> which lock already depends on the new lock.
>>>    :
>>>   Possible unsafe locking scenario:
>>>
>>>         CPU0                    CPU1
>>>         ----                    ----
>>>    lock(sb_internal);
>>>                                 lock(fs_reclaim);
>>>                                 lock(sb_internal);
>>>    lock(fs_reclaim);
>>>
>>>   *** DEADLOCK ***
>>>
>>> 4 locks held by fsfreeze/4346:
>>>   #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650
>>>   #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at: freeze_super+0xda/0x290
>>>   #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650
>>>   #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>>
>>> stack backtrace:
>>> Call Trace:
>>>   dump_stack+0xe0/0x19a
>>>   print_circular_bug.isra.10.cold.34+0x2f4/0x435
>>>   check_prev_add.constprop.19+0xca1/0x15f0
>>>   validate_chain.isra.14+0x11af/0x3b50
>>>   __lock_acquire+0x728/0x1200
>>>   lock_acquire+0x269/0x5a0
>>>   fs_reclaim_acquire.part.19+0x29/0x30
>>>   fs_reclaim_acquire+0x19/0x20
>>>   kmem_cache_alloc+0x3e/0x3f0
>>>   kmem_zone_alloc+0x79/0x150
>>>   xfs_trans_alloc+0xfa/0x9d0
>>>   xfs_sync_sb+0x86/0x170
>>>   xfs_log_sbcount+0x10f/0x140
>>>   xfs_quiesce_attr+0x134/0x270
>>>   xfs_fs_freeze+0x4a/0x70
>>>   freeze_super+0x1af/0x290
>>>   do_vfs_ioctl+0xedc/0x16c0
>>>   ksys_ioctl+0x41/0x80
>>>   __x64_sys_ioctl+0x73/0xa9
>>>   do_syscall_64+0x18f/0xd23
>>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>
>>> This is a false positive as all the dirty pages are flushed out before
>>> the filesystem can be frozen.
>>>
>>> One way to avoid this splat is to add GFP_NOFS to the affected allocation
>>> calls by using the memalloc_nofs_save()/memalloc_nofs_restore() pair.
>>> This shouldn't matter unless the system is really running out of memory.
>>> In that particular case, the filesystem freeze operation may fail while
>>> it was succeeding previously.
>>>
>>> Without this patch, the command sequence below will show that the lock
>>> dependency chain sb_internal -> fs_reclaim exists.
>>>
>>>   # fsfreeze -f /home
>>>   # fsfreeze --unfreeze /home
>>>   # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal
>>>
>>> After applying the patch, such sb_internal -> fs_reclaim lock dependency
>>> chain can no longer be found. Because of that, the locking dependency
>>> warning will not be shown.
>>>
>>> Suggested-by: Dave Chinner <david@fromorbit.com>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>> Looks good to me,
>> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
>>
>> Will this patch be merged into the xfs tree soon?
> It should appear in for-next in the next day or so.  I am trying to push
> there only every other couple of weeks to reduce the amount of developer
> tree rebasing that has to go on when people are trying to land a complex
> series.
>
> --D

Thanks for the clarification.

Cheers,
Longman