Messages in this thread | | | Subject | Re: [PATCH v3] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim | From | Waiman Long <> | Date | Thu, 18 Jun 2020 11:36:55 -0400 |
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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@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
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 >>
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