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Date:   Thu, 18 Jun 2020 08:20:51 -0700
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>
Subject: Re: [PATCH v3] xfs: Fix false positive lockdep warning with
 sb_internal & fs_reclaim
Message-ID: <20200618152051.GU11245@magnolia>
References: <20200618150557.23741-1-longman@redhat.com>
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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@fromorbit.com>
> Signed-off-by: Waiman Long <longman@redhat.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

>  
> +	/*
> +	 * 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
>