Re: [PATCH 2/2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim
From: Waiman Long
Date: Mon Jun 15 2020 - 16:53:56 EST
On 6/15/20 12:43 PM, Darrick J. Wong wrote:
On Mon, Jun 15, 2020 at 12:08:30PM -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.
Perhaps breaking the fs_reclaim pseudo lock into a per filesystem lock
may fix the issue. However, that will greatly complicate the logic and
may not be worth it.
Another way to fix it is to disable the taking of the fs_reclaim
pseudo lock when in the freezing code path as a reclaim on the
freezed filesystem is not possible. By using the newly introduced
PF_MEMALLOC_NOLOCKDEP flag, lockdep checking is disabled in
xfs_trans_alloc() if XFS_TRANS_NO_WRITECOUNT flag is set.
In the freezing path, there is another path where memory allocation
is being done without the XFS_TRANS_NO_WRITECOUNT flag:
xfs_fs_freeze()
=> xfs_quiesce_attr()
=> xfs_log_quiesce()
=> xfs_log_unmount_write()
=> xlog_unmount_write()
=> xfs_log_reserve()
=> xlog_ticket_alloc()
In this case, we just disable fs reclaim for this particular 600 bytes
memory allocation.
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>
---
fs/xfs/xfs_log.c | 9 +++++++++
fs/xfs/xfs_trans.c | 30 ++++++++++++++++++++++++++----
2 files changed, 35 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 00fda2e8e738..33244680d0d4 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -830,8 +830,17 @@ xlog_unmount_write(
xfs_lsn_t lsn;
uint flags = XLOG_UNMOUNT_TRANS;
int error;
+ unsigned long pflags;
+ /*
+ * xfs_log_reserve() allocates memory. This can lead to fs reclaim
+ * which may conflicts with the unmount process. To avoid that,
+ * disable fs reclaim for this allocation.
+ */
+ current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS);
error = xfs_log_reserve(mp, 600, 1, &tic, XFS_LOG, 0);
+ current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS);
+
if (error)
goto out_err;
diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index 3c94e5ff4316..ddb10ad3f51f 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -255,7 +255,27 @@ xfs_trans_alloc(
struct xfs_trans **tpp)
{
struct xfs_trans *tp;
- int error;
+ int error = 0;
+ unsigned long pflags = -1;
+
+ /*
+ * When XFS_TRANS_NO_WRITECOUNT is set, it means there are no dirty
+ * data pages in the filesystem at this point.
That's not true. Look at the other callers of xfs_trans_alloc_empty.
Yes, I am aware of that. I can change it to check the freeze state.
Also: Why not set PF_MEMALLOC_NOFS at the start of the freeze call
chain?
I guess we can do that, but it eliminates a potential source for memory
reclaim leading to freeze error when not much free memory is left. We
can go this route if you think this is not a problem.
Cheers,
Longman