blktap2: fix synchronization in blktap_device_run_queue()

c/s 896 (use of blk_rq_map_sg()) made the problem worse, but from what
I can tell there had been races (ring and stats updates) before. If
that's not a correct observation, a perhaps better solution might be
to move the struct scatterlist array out of struct blktap (and make it
e.g. an on-stack variable, the problem being that
blktap_device_process_request() has a pretty large stack frame
already - shrinking this might be possible by moving e.g. the
struct blktap_grant_table and struct blkif_request blkif_req instances
the other way if the locking change here is the right thing to do).

Signed-off-by: Jan Beulich <jbeulich@novell.com>

diff --git a/drivers/xen/blktap2/device.c b/drivers/xen/blktap2/device.c
index 6424994cfe70cfa6d388125c8024c2c8626b750f..911fac3e1fefea87df72abbe1b9b2c29c6cac962 100644 (file)
--- a/drivers/xen/blktap2/device.c
+++ b/drivers/xen/blktap2/device.c
@@ -888,7 +888,7 @@ blktap_device_run_queue(struct blktap *tap)
                blkdev_dequeue_request(req);
 
                spin_unlock_irq(&dev->lock);
-               down_read(&tap->tap_sem);
+               down_write(&tap->tap_sem);
 
                err = blktap_device_process_request(tap, request, req);
                if (!err)
@@ -898,7 +898,7 @@ blktap_device_run_queue(struct blktap *tap)
                        blktap_request_free(tap, request);
                }
 
-               up_read(&tap->tap_sem);
+               up_write(&tap->tap_sem);
                spin_lock_irq(&dev->lock);
        }