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SubjectRe: [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>

--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
>

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