Messages in this thread |  | | Subject | Re: [PATCH v6] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim | From | Waiman Long <> | Date | Mon, 20 Jul 2020 11:46:56 -0400 |
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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@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> >> 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@fromorbit.com> >>> Signed-off-by: Waiman Long <longman@redhat.com> >> Looks good to me, >> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> >> >> 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
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