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
