Patch "xfs: use dedicated log worker wq to avoid deadlock with cil wq" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

xfs: use dedicated log worker wq to avoid deadlock with cil wq

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xfs-use-dedicated-log-worker-wq-to-avoid-deadlock-with-cil-wq.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Brian Foster <[email protected]>
Date: Tue, 28 Mar 2017 14:51:44 -0700
Subject: xfs: use dedicated log worker wq to avoid deadlock with cil wq

From: Brian Foster <[email protected]>

commit 696a562072e3c14bcd13ae5acc19cdf27679e865 upstream.

The log covering background task used to be part of the xfssyncd workqueue. That workqueue was removed as of commit 5889608df ("xfs:
syncd workqueue is no more") and the associated work item scheduled to the xfs-log wq. The latter is used for log buffer I/O completion.

Since xfs_log_worker() can invoke a log flush, a deadlock is possible between the xfs-log and xfs-cil workqueues. Consider the following codepath from xfs_log_worker():

xfs_log_worker()
xfs_log_force()

    _xfs_log_force()
      xlog_cil_force()
        xlog_cil_force_lsn()
          xlog_cil_push_now()
            flush_work()

The above is in xfs-log wq context and blocked waiting on the completion of an xfs-cil work item. Concurrently, the cil push in progress can end up blocked here:

xlog_cil_push_work()
xlog_cil_push()

    xlog_write()
      xlog_state_get_iclog_space()
        xlog_wait(&log->l_flush_wait, ...)

The above is in xfs-cil context waiting on log buffer I/O completion, which executes in xfs-log wq context. In this scenario both workqueues are deadlocked waiting on eachother.

Add a new workqueue specifically for the high level log covering and ail pushing worker, as was the case prior to commit 5889608df.

---
 fs/xfs/xfs_log.c   |    2 +-
 fs/xfs/xfs_mount.h |    1 +
 fs/xfs/xfs_super.c |    8 ++++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

@@ -1293,7 +1293,7 @@ void
 xfs_log_work_queue(
 	struct xfs_mount        *mp)
 {
-	queue_delayed_work(mp->m_log_workqueue, &mp->m_log->l_work,
+	queue_delayed_work(mp->m_sync_workqueue, &mp->m_log->l_work,
 				msecs_to_jiffies(xfs_syncd_centisecs * 10));
 }
 

@@ -183,6 +183,7 @@ typedef struct xfs_mount {
 	struct workqueue_struct	*m_reclaim_workqueue;
 	struct workqueue_struct	*m_log_workqueue;
 	struct workqueue_struct *m_eofblocks_workqueue;
+	struct workqueue_struct	*m_sync_workqueue;
 
 	/*
 	 * Generation of the filesysyem layout.  This is incremented by each

@@ -877,8 +877,15 @@ xfs_init_mount_workqueues(
 	if (!mp->m_eofblocks_workqueue)
 		goto out_destroy_log;
 
+	mp->m_sync_workqueue = alloc_workqueue("xfs-sync/%s", WQ_FREEZABLE, 0,
+					       mp->m_fsname);
+	if (!mp->m_sync_workqueue)
+		goto out_destroy_eofb;
+
 	return 0;
 
+out_destroy_eofb:
+	destroy_workqueue(mp->m_eofblocks_workqueue);
 out_destroy_log:
 	destroy_workqueue(mp->m_log_workqueue);
 out_destroy_reclaim:

@@ -899,6 +906,7 @@ STATIC void
 xfs_destroy_mount_workqueues(
 	struct xfs_mount	*mp)
 {
+	destroy_workqueue(mp->m_sync_workqueue);
 	destroy_workqueue(mp->m_eofblocks_workqueue);
 	destroy_workqueue(mp->m_log_workqueue);
 	destroy_workqueue(mp->m_reclaim_workqueue);

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/xfs-fix-missed-holes-in-seek_hole-implementation.patch queue-4.11/xfs-use-dedicated-log-worker-wq-to-avoid-deadlock-with-cil-wq.patch queue-4.11/xfs-use-b_state-to-fix-buffer-i-o-accounting-release-race.patch