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Re: [PATCH] xfs: force log before unmount

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: force log before unmount
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Fri, 22 Jun 2012 15:39:35 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120621235058.GF10673@dastard>
References: <20120621192541.630212563@xxxxxxx> <20120621192600.936441949@xxxxxxx> <20120621235058.GF10673@dastard>
User-agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:9.0) Gecko/20120122 Thunderbird/9.0
On 06/21/12 18:50, Dave Chinner wrote:
On Thu, Jun 21, 2012 at 02:25:42PM -0500, tinguely@xxxxxxx wrote:
From: Mark Tinguely<tinguely@xxxxxxx>

Testing has found a few crashes of dereference of NULL in the
xlog_assign_tail_lsn_locked() and xlog_get_lowest_lsn() routines from
buf iodone callbacks. The filesystem is be shutdown. At the point of
the crash, the log has alreadly been freed which leads to the NULL pointer
dereference.

One crash still pointed to the buffer that was going through the iodone
path and that buffer had XFS_SB_MAGIC in it.

Which points to the problem, but you haven't quite worked out what
the root cause of this is so the patch doesn't actually fix the
problem, It changes the timing of IO, so it can certainly mke it
seem like it fixed the race condition....

This patch sits on top of Ben's proposed patch to shutdown the sync worker.
Although it did not cause the crash I would like to be on the safe side, and
perform the shutdown of the sync worker to occur before the last write of the
superblock.

The sync worker is not an actor in this race condition - where it
was placed in xfs_log_unmount() is the correct place for it.

Then force the log to get the superblock buffer into the AIL before
it is pushed for the last time..

The superblock is already in the AIL - otherwise we wouldn't have
crashed trying to remove it from the AIL after the log has been torn
down.

To the best of my searching, xfs_log_unmount_write() should not dirty the
superblock..

It doesn't - it just writes an unmount record to the log.

Signed-off-by: Mark Tinguely<tinguely@xxxxxxx>
---
  fs/xfs/xfs_log.c   |    1 -
  fs/xfs/xfs_mount.c |    2 ++
  2 files changed, 2 insertions(+), 1 deletion(-)

Index: b/fs/xfs/xfs_log.c
===================================================================
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -810,7 +810,6 @@ xfs_log_unmount_write(xfs_mount_t *mp)
  void
  xfs_log_unmount(xfs_mount_t *mp)
  {
-       cancel_delayed_work_sync(&mp->m_sync_work);
        xfs_trans_ail_destroy(mp);
        xlog_dealloc_log(mp->m_log);
  }
Index: b/fs/xfs/xfs_mount.c
===================================================================
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1517,6 +1517,7 @@ xfs_unmountfs(
                xfs_warn(mp, "Unable to free reserved block pool. "
                                "Freespace may not be correct on next mount.");

+       cancel_delayed_work_sync(&mp->m_sync_work);
        error = xfs_log_sbcount(mp);
        if (error)
                xfs_warn(mp, "Unable to update superblock counters. "
@@ -1526,6 +1527,7 @@ xfs_unmountfs(
         * At this point we might have modified the superblock again and thus
         * added an item to the AIL, thus flush it again.
         */
+       xfs_log_force(mp, XFS_LOG_SYNC);

This is a no-op - xfs_log_sbcount() does a synchronous transaction,
so the log has already been forced at this point and the superblock
is not pinned in memory.

        xfs_ail_push_all_sync(mp->m_ail);
        xfs_wait_buftarg(mp->m_ddev_targp);

So, what is different about the superblock buffer? It's an uncached
buffer. What does that mean? It means that when xfs_wait_buftarg()
walks the LRU, it never finds the superblock buffer because uncached
buffers are never put on the LRU.


Thanks Dave. Just interested; are the uncached buffers kept off the
LRU with an extra b_hold reference?

Hence, when xfs_ail_push_all_sync() triggers an async write of the
superblock, nobody ever waits for it to complete. Hence the log and
AIL are torn down, and when the IO completes it goes to remove it
from the AIL (which succeeds) and then move the log tail because it
was the only item in the AIL, it goes kaboom.

So the root cause is that we are not waiting for the superblock IO
to complete i.e. it's not at all related to the xfs_sync_worker(), log
forces, etc. What we really need to do is after the
xfs_log_sbcount() call is write the superblock synchronously, and we
can do that in xfs_log_sbcount() because all the callers of
xfs_log_sbcount() have this same problem of not waiting for the SB
to be written correctly. i.e. add the guts of xfs_sync_fsdata() to
xfs_log_sbcount()....



I did not expect the sync worker was the cause. Ben's patch is not even
in at least one of the crashes. I thought it would be good thing to have
the sync worker idle by the time we write the unmount record in the log.

Thank-you again for the lesson and insight.

--Mark Tinguely.

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