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[PATCH] xfs: fix double free of trans in log recovery on I/O error

To: xfs@xxxxxxxxxxx
Subject: [PATCH] xfs: fix double free of trans in log recovery on I/O error
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Thu, 21 Aug 2014 15:18:12 -0400
Cc: david@xxxxxxxxxxxxx, alex@xxxxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
XFS log recovery builds up an xlog_recover object as it passes through
the log operations on the physical log. These structures are managed via
a hash table and are allocated when a new transaction is encountered and
freed once a commit operation for the transaction is encountered.

This state machine for active transactions is implemented by a
combination of xlog_do_recovery_pass(), which walks through the log
buffers and xlog_recover_process_data() which processes log operations
within each buffer. The latter function decides whether to allocate a
new xlog_recover, add to it or commit and ultimately free it.  If an
error occurs at any point during the lifecycle of a particular
xlog_recover, xlog_recover_process_data() frees the object and returns
an error.

xlog_recover_commit_trans() handles the final processing of the
transaction. It submits whatever I/O is required for the transaction and
frees xlog_recover object along with the transaction items it tracks. If
an error occurs at the final stages of the commit operation, such as I/O
failure, both xlog_recover_commit_trans() and
xlog_recover_process_data() attempt to free the trans object.

Modify xlog_recover_commit_trans() to only free the trans object on
successful completion of the trans, including any I/O errors that might
occur when recovering the log.

Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>

Hi all,

I found that the recent buffer I/O rework fixes didn't address the crash
reproduced by the dm-flakey/log recovery test case I posted recently. I
tracked the crash down to this, which allows the test to pass. This
addresses the crash I saw when running the reproducer manually with the
metadump that Alex posted as well.

FWIW, I also went back and tested the xfs_buf_iowait() experiment in
both scenarios (Alex's metadump and xfstests test) and they all
reproduce the same crash for me. I think that either I'm still not
reproducing the original problem, something else might have contaminated
the original xfs_buf_iowait() test to give a false positive, or
something else entirely is going on.


If you have a chance, I think it might be interesting to see whether you
reproduce any problems with this patch. It looks like this is a
regression introduced by:

        2a84108f xfs: free the list of recovery items on error

... but I have no idea if that's in whatever kernel you're running.


 fs/xfs/xfs_log_recover.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 176c4b3..daca9a6 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -3528,10 +3528,15 @@ out:
        if (!list_empty(&done_list))
                list_splice_init(&done_list, &trans->r_itemq);
-       xlog_recover_free_trans(trans);
        error2 = xfs_buf_delwri_submit(&buffer_list);
-       return error ? error : error2;
+       if (!error)
+               error = error2;
+       /* caller frees trans on error */
+       if (!error)
+               xlog_recover_free_trans(trans);
+       return error;

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