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

[Waiman Long]

On 7/20/20 11:40 AM, Darrick J. Wong wrote:
> On Mon, Jul 20, 2020 at 11:32:03AM -0400, Waiman Long wrote:
> > ----- Original Message -----
> > From: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
> > To: "Waiman Long" <longman@xxxxxxxxxx>
> > Cc: linux-xfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, "Dave Chinner" <david@xxxxxxxxxxxxx>, "Qian Cai" <cai@xxxxxx>, "Eric Sandeen" <sandeen@xxxxxxxxxx>
> > Sent: Monday, July 13, 2020 12:41:12 PM
> > Subject: Re: [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@xxxxxxxxxxxxx>
> > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Looks good to me,
> > Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> >
> > Will this patch be merged into the xfs tree soon?
> It should appear in for-next in the next day or so.  I am trying to push
> there only every other couple of weeks to reduce the amount of developer
> tree rebasing that has to go on when people are trying to land a complex
> series.
>
> --D

Thanks for the clarification.

Cheers,
Longman