[PATCH 1/2] xfs: wake commit waiters on CIL abort before log item abort

[Brian Foster <bfoster@redhat.com>]

XFS shutdown deadlocks have been reproduced by fstest generic/475.
The deadlock signature involves log I/O completion running error
handling to abort logged items and waiting for an inode cluster
buffer lock in the buffer item unpin handler. The buffer lock is
held by xfsaild attempting to flush an inode. The buffer happens to
be pinned and so xfs_iflush() triggers an async log force to begin
work required to get it unpinned. The log force is blocked waiting
on the commit completion, which never occurs and thus leaves the
filesystem deadlocked.

The root problem is that aborted log I/O completion pots commit
completion behind callback completion, which is unexpected for async
log forces. Under normal running conditions, an async log force
returns to the caller once the CIL ctx has been formatted/submitted
and the commit completion event triggered at the tail end of
xlog_cil_push(). If the filesystem has shutdown, however, we rely on
xlog_cil_committed() to trigger the completion event and it happens
to do so after running log item unpin callbacks. This makes it
unsafe to invoke an async log force from contexts that hold locks
that might also be required in log completion processing.

To address this problem, wake commit completion waiters before
aborting log items in the log I/O completion handler. This ensures
that an async log force will not deadlock on held locks if the
filesystem happens to shutdown. Note that it is still unsafe to
issue a sync log force while holding such locks because a sync log
force explicitly waits on the force completion, which occurs after
log I/O completion processing.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/xfs_log_cil.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index d3884e08b43c..5e595948bc5a 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -582,6 +582,19 @@ xlog_cil_committed(
 	struct xfs_cil_ctx	*ctx = args;
 	struct xfs_mount	*mp = ctx->cil->xc_log->l_mp;
 
+	/*
+	 * If the I/O failed, we're aborting the commit and already shutdown.
+	 * Wake any commit waiters before aborting the log items so we don't
+	 * block async log pushers on callbacks. Async log pushers explicitly do
+	 * not wait on log force completion because they may be holding locks
+	 * required to unpin items.
+	 */
+	if (abort) {
+		spin_lock(&ctx->cil->xc_push_lock);
+		wake_up_all(&ctx->cil->xc_commit_wait);
+		spin_unlock(&ctx->cil->xc_push_lock);
+	}
+
 	xfs_trans_committed_bulk(ctx->cil->xc_log->l_ailp, ctx->lv_chain,
 					ctx->start_lsn, abort);
 
@@ -589,15 +602,7 @@ xlog_cil_committed(
 	xfs_extent_busy_clear(mp, &ctx->busy_extents,
 			     (mp->m_flags & XFS_MOUNT_DISCARD) && !abort);
 
-	/*
-	 * If we are aborting the commit, wake up anyone waiting on the
-	 * committing list.  If we don't, then a shutdown we can leave processes
-	 * waiting in xlog_cil_force_lsn() waiting on a sequence commit that
-	 * will never happen because we aborted it.
-	 */
 	spin_lock(&ctx->cil->xc_push_lock);
-	if (abort)
-		wake_up_all(&ctx->cil->xc_commit_wait);
 	list_del(&ctx->committing);
 	spin_unlock(&ctx->cil->xc_push_lock);
 
-- 
2.17.2