[PATCH v2] xfs: buffer ->bi_end_io function requires irq-safe lock

[Brian Foster <bfoster@xxxxxxxxxx>]

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@xxxxxxxxxx>
---

v2:
- Use cmpxchg() instead of lock. [hch]
v1: http://oss.sgi.com/pipermail/xfs/2016-April/048384.html

 fs/xfs/xfs_buf.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 9a2191b..e71cfbd 100644
--- a/fs/xfs/xfs_buf.c
+++ b/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));
-- 
2.4.11