xfs: log head and tail aren't reliable during shutdown

I'm seeing assert failures from xlog_space_left() after a shutdown
has begun that look like:

XFS (dm-0): log I/O error -5
XFS (dm-0): xfs_do_force_shutdown(0x2) called from line 1338 of file fs/xfs/xfs_log.c. Return address = xlog_ioend_work+0x64/0xc0
XFS (dm-0): Log I/O Error Detected.
XFS (dm-0): Shutting down filesystem. Please unmount the filesystem and rectify the problem(s)
XFS (dm-0): xlog_space_left: head behind tail
XFS (dm-0):   tail_cycle = 6, tail_bytes = 2706944
XFS (dm-0):   GH   cycle = 6, GH   bytes = 1633867
XFS: Assertion failed: 0, file: fs/xfs/xfs_log.c, line: 1310
------------[ cut here ]------------
Call Trace:
 xlog_space_left+0xc3/0x110
 xlog_grant_push_threshold+0x3f/0xf0
 xlog_grant_push_ail+0x12/0x40
 xfs_log_reserve+0xd2/0x270
 ? __might_sleep+0x4b/0x80
 xfs_trans_reserve+0x18b/0x260
.....

There are two things here. Firstly, after a shutdown, the log head
and tail can be out of whack as things abort and release (or don't
release) resources, so checking them for sanity doesn't make much
sense. Secondly, xfs_log_reserve() can race with shutdown and so it
can still fail like this even though it has already checked for a
log shutdown before calling xlog_grant_push_ail().

So, before ASSERT failing in xlog_space_left(), make sure we haven't
already shut down....
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_log.c

@@ -1307,16 +1307,18 @@ xlog_assign_tail_lsn(
  * wrap the tail, we should blow up.  Rather than catch this case here,
  * we depend on other ASSERTions in other parts of the code.   XXXmiken
  *
- * This code also handles the case where the reservation head is behind
- * the tail.  The details of this case are described below, but the end
- * result is that we return the size of the log as the amount of space left.
+ * If reservation head is behind the tail, we have a problem. Warn about it,
+ * but then treat it as if the log is empty.
+ *
+ * If the log is shut down, the head and tail may be invalid or out of whack, so
+ * shortcut invalidity asserts in this case so that we don't trigger them
+ * falsely.
  */
 STATIC int
 xlog_space_left(
 	struct xlog	*log,
 	atomic64_t	*head)
 {
-	int		free_bytes;
 	int		tail_bytes;
 	int		tail_cycle;
 	int		head_cycle;
@@ -1326,29 +1328,30 @@ xlog_space_left(
 	xlog_crack_atomic_lsn(&log->l_tail_lsn, &tail_cycle, &tail_bytes);
 	tail_bytes = BBTOB(tail_bytes);
 	if (tail_cycle == head_cycle && head_bytes >= tail_bytes)
-		free_bytes = log->l_logsize - (head_bytes - tail_bytes);
-	else if (tail_cycle + 1 < head_cycle)
+		return log->l_logsize - (head_bytes - tail_bytes);
+	if (tail_cycle + 1 < head_cycle)
 		return 0;
-	else if (tail_cycle < head_cycle) {
+
+	/* Ignore potential inconsistency when shutdown. */
+	if (xlog_is_shutdown(log))
+		return log->l_logsize;
+
+	if (tail_cycle < head_cycle) {
 		ASSERT(tail_cycle == (head_cycle - 1));
-		free_bytes = tail_bytes - head_bytes;
-	} else {
-		/*
-		 * The reservation head is behind the tail.
-		 * In this case we just want to return the size of the
-		 * log as the amount of space left.
-		 */
-		xfs_alert(log->l_mp, "xlog_space_left: head behind tail");
-		xfs_alert(log->l_mp,
-			  "  tail_cycle = %d, tail_bytes = %d",
-			  tail_cycle, tail_bytes);
-		xfs_alert(log->l_mp,
-			  "  GH   cycle = %d, GH   bytes = %d",
-			  head_cycle, head_bytes);
-		ASSERT(0);
-		free_bytes = log->l_logsize;
+		return tail_bytes - head_bytes;
 	}
-	return free_bytes;
+
+	/*
+	 * The reservation head is behind the tail. In this case we just want to
+	 * return the size of the log as the amount of space left.
+	 */
+	xfs_alert(log->l_mp, "xlog_space_left: head behind tail");
+	xfs_alert(log->l_mp, "  tail_cycle = %d, tail_bytes = %d",
+		  tail_cycle, tail_bytes);
+	xfs_alert(log->l_mp, "  GH   cycle = %d, GH   bytes = %d",
+		  head_cycle, head_bytes);
+	ASSERT(0);
+	return log->l_logsize;
 }