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

["Darrick J. Wong" <darrick.wong@oracle.com>]

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>

--D

> ---
>  fs/xfs/xfs_super.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 379cbff438bc..0797a96b83d6 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -913,11 +913,21 @@ xfs_fs_freeze(
>  	struct super_block	*sb)
>  {
>  	struct xfs_mount	*mp = XFS_M(sb);
> +	unsigned int		flags;
> +	int			ret;
>  
> +	/*
> +	 * The filesystem is now frozen far enough that memory reclaim
> +	 * cannot safely operate on the filesystem. Hence we need to
> +	 * set a GFP_NOFS context here to avoid recursion deadlocks.
> +	 */
> +	flags = memalloc_nofs_save();
>  	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);
> +	memalloc_nofs_restore(flags);
> +	return ret;
>  }
>  
>  STATIC int
> -- 
> 2.18.1
>