xfs: buffer ->bi_end_io function requires irq-safe lock

Reports have surfaced of a lockdep splat complaining about an
irq-safe -> irq-unsafe locking order in the xfs_buf_bio_end_io() bio
completion handler. This only occurs when I/O errors are present
because bp->b_lock is only acquired in this context to protect
setting an error on the buffer. The problem is that this lock can be
acquired with the (request_queue) q->queue_lock held. See
scsi_end_request() or ata_qc_schedule_eh(), for example.

Replace the locked test/set of b_io_error with a cmpxchg() call.
This eliminates the need for the lock and thus the lock ordering
problem goes away.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_buf.c

@@ -1100,22 +1100,18 @@ xfs_bwrite(
 	return error;
 }
 
-STATIC void
+static void
 xfs_buf_bio_end_io(
 	struct bio		*bio)
 {
-	xfs_buf_t		*bp = (xfs_buf_t *)bio->bi_private;
+	struct xfs_buf		*bp = (struct xfs_buf *)bio->bi_private;
 
 	/*
 	 * don't overwrite existing errors - otherwise we can lose errors on
 	 * buffers that require multiple bios to complete.
 	 */
-	if (bio->bi_error) {
-		spin_lock(&bp->b_lock);
-		if (!bp->b_io_error)
-			bp->b_io_error = bio->bi_error;
-		spin_unlock(&bp->b_lock);
-	}
+	if (bio->bi_error)
+		cmpxchg(&bp->b_io_error, 0, bio->bi_error);
 
 	if (!bp->b_error && xfs_buf_is_vmapped(bp) && (bp->b_flags & XBF_READ))
 		invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));