xfs: check buffer pin state after locking in delwri_submit

AIL flushing can get stuck here:

[316649.005769] INFO: task xfsaild/pmem1:324525 blocked for more than 123 seconds.
[316649.007807]       Not tainted 5.17.0-rc6-dgc+ #975
[316649.009186] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[316649.011720] task:xfsaild/pmem1   state:D stack:14544 pid:324525 ppid:     2 flags:0x00004000
[316649.014112] Call Trace:
[316649.014841]  <TASK>
[316649.015492]  __schedule+0x30d/0x9e0
[316649.017745]  schedule+0x55/0xd0
[316649.018681]  io_schedule+0x4b/0x80
[316649.019683]  xfs_buf_wait_unpin+0x9e/0xf0
[316649.021850]  __xfs_buf_submit+0x14a/0x230
[316649.023033]  xfs_buf_delwri_submit_buffers+0x107/0x280
[316649.024511]  xfs_buf_delwri_submit_nowait+0x10/0x20
[316649.025931]  xfsaild+0x27e/0x9d0
[316649.028283]  kthread+0xf6/0x120
[316649.030602]  ret_from_fork+0x1f/0x30

in the situation where flushing gets preempted between the unpin
check and the buffer trylock under nowait conditions:

	blk_start_plug(&plug);
	list_for_each_entry_safe(bp, n, buffer_list, b_list) {
		if (!wait_list) {
			if (xfs_buf_ispinned(bp)) {
				pinned++;
				continue;
			}
Here >>>>>>
			if (!xfs_buf_trylock(bp))
				continue;

This means submission is stuck until something else triggers a log
force to unpin the buffer.

To get onto the delwri list to begin with, the buffer pin state has
already been checked, and hence it's relatively rare we get a race
between flushing and encountering a pinned buffer in delwri
submission to begin with. Further, to increase the pin count the
buffer has to be locked, so the only way we can hit this race
without failing the trylock is to be preempted between the pincount
check seeing zero and the trylock being run.

Hence to avoid this problem, just invert the order of trylock vs
pin check. We shouldn't hit that many pinned buffers here, so
optimising away the trylock for pinned buffers should not matter for
performance at all.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_buf.c

@@ -2094,12 +2094,13 @@ xfs_buf_delwri_submit_buffers(
 	blk_start_plug(&plug);
 	list_for_each_entry_safe(bp, n, buffer_list, b_list) {
 		if (!wait_list) {
+			if (!xfs_buf_trylock(bp))
+				continue;
 			if (xfs_buf_ispinned(bp)) {
+				xfs_buf_unlock(bp);
 				pinned++;
 				continue;
 			}
-			if (!xfs_buf_trylock(bp))
-				continue;
 		} else {
 			xfs_buf_lock(bp);
 		}