linux-kernel - Re: [PATCH v3] xfs: Fix false positive lockdep warning with sb_internal & fs

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Date:   Thu, 18 Jun 2020 11:36:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave Chinner <david@...morbit.com>, Qian Cai <cai@....pw>,
        Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH v3] xfs: Fix false positive lockdep warning with
 sb_internal & fs_reclaim

On 6/18/20 11:20 AM, Darrick J. Wong wrote:
> On Thu, Jun 18, 2020 at 11:05:57AM -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. This is what PF_MEMALLOC_NOFS per-process flag is for. This does
>> reduce the potential source of memory where reclaim can be done. This
>> shouldn't really 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@...morbit.com>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   fs/xfs/xfs_super.c | 24 +++++++++++++++++++++++-
>>   1 file changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
>> index 379cbff438bc..6a95c82f2f1b 100644
>> --- a/fs/xfs/xfs_super.c
>> +++ b/fs/xfs/xfs_super.c
>> @@ -913,11 +913,33 @@ xfs_fs_freeze(
>>   	struct super_block	*sb)
>>   {
>>   	struct xfs_mount	*mp = XFS_M(sb);
>> +	unsigned long pflags;
>> +	int ret;
> Minor nit: please indent the variable names to line up with *sb/*mp.
>
> Otherwise this seems reasoanble.
>
> --D

Yes, I should have done that.

Will send out another version.

Thanks,
Longman

>>   
>> +	/*
>> +	 * A fs_reclaim pseudo lock is added to check for potential deadlock
>> +	 * condition with fs reclaim. The following lockdep splat was hit
>> +	 * occasionally. This is actually a false positive as the allocation
>> +	 * is being done only after the frozen filesystem is no longer dirty.
>> +	 * One way to avoid this splat is to add GFP_NOFS to the affected
>> +	 * allocation calls. This is what PF_MEMALLOC_NOFS is for.
>> +	 *
>> +	 *       CPU0                    CPU1
>> +	 *       ----                    ----
>> +	 *  lock(sb_internal);
>> +	 *                               lock(fs_reclaim);
>> +	 *                               lock(sb_internal);
>> +	 *  lock(fs_reclaim);
>> +	 *
>> +	 *  *** DEADLOCK ***
>> +	 */
>> +	current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS);
>>   	xfs_stop_block_reaping(mp);
>>   	xfs_save_resvblks(mp);
>>   	xfs_quiesce_attr(mp);
>> -	return xfs_sync_sb(mp, true);
>> +	ret = xfs_sync_sb(mp, true);
>> +	current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS);
>> +	return ret;
>>   }
>>   
>>   STATIC int
>> -- 
>> 2.18.1
>>