[PATCH 2/2] virtio_scsi: remove smp_read_barrier_depends

Ming Lei
2014-05-07T14:13:21Z

The barrier in virtscsi_req_done was introduced to
order reading req_vq and decreasing tgt->reqs, but
it isn't needed now because req_vq is read from
scsi->req_vqs[vq->index - VIRTIO_SCSI_VQ_BASE]
instead of tgt->req_vq, so remove the unnecessary
barrier.

Also remove related comment about the barrier.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
drivers/scsi/virtio_scsi.c | 52 +++-----------------------------------------
1 file changed, 3 insertions(+), 49 deletions(-)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index aead20f..0c77ed6 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -73,19 +73,9 @@ struct virtio_scsi_vq {
* queue, and also lets the driver optimize the IRQ affinity for the virtqueues
* (each virtqueue's affinity is set to the CPU that "owns" the queue).
*
- * An interesting effect of this policy is that only writes to req_vq need to
- * take the tgt_lock. Read can be done outside the lock because:
- *
- * - writes of req_vq only occur when atomic_inc_return(&tgt->reqs) returns 1.
- * In that case, no other CPU is reading req_vq: even if they were in
- * virtscsi_queuecommand_multi, they would be spinning on tgt_lock.
- *
- * - reads of req_vq only occur when the target is not idle (reqs != 0).
- * A CPU that enters virtscsi_queuecommand_multi will not modify req_vq.
- *
- * Similarly, decrements of reqs are never concurrent with writes of req_vq.
- * Thus they can happen outside the tgt_lock, provided of course we make reqs
- * an atomic_t.
+ * tgt_lock is held to serialize reading and writing req_vq, and reading
+ * req_vq should have been done concurrently, but need a bit complicated
+ * trick.
*/
struct virtio_scsi_target_state {
/* This spinlock never held at the same time as vq_lock. */
@@ -238,38 +228,6 @@ static void virtscsi_req_done(struct virtqueue *vq)
int index = vq->index - VIRTIO_SCSI_VQ_BASE;
struct virtio_scsi_vq *req_vq = &vscsi->req_vqs[index];

- /*
- * Read req_vq before decrementing the reqs field in
- * virtscsi_complete_cmd.
- *
- * With barriers:
- *
- * CPU #0 virtscsi_queuecommand_multi (CPU #1)
- * ------------------------------------------------------------
- * lock vq_lock
- * read req_vq
- * read reqs (reqs = 1)
- * write reqs (reqs = 0)
- * increment reqs (reqs = 1)
- * write req_vq
- *
- * Possible reordering without barriers:
- *
- * CPU #0 virtscsi_queuecommand_multi (CPU #1)
- * ------------------------------------------------------------
- * lock vq_lock
- * read reqs (reqs = 1)
- * write reqs (reqs = 0)
- * increment reqs (reqs = 1)
- * write req_vq
- * read (wrong) req_vq
- *
- * We do not need a full smp_rmb, because req_vq is required to get
- * to tgt->reqs: tgt is &vscsi->tgt[sc->device->id], where sc is stored
- * in the virtqueue as the user token.
- */
- smp_read_barrier_depends();
-
virtscsi_vq_done(vscsi, req_vq, virtscsi_complete_cmd);
};

@@ -560,10 +518,6 @@ static struct virtio_scsi_vq *virtscsi_pick_vq(struct virtio_scsi *vscsi,

spin_lock_irqsave(&tgt->tgt_lock, flags);

- /*
- * The memory barrier after atomic_inc_return matches
- * the smp_read_barrier_depends() in virtscsi_req_done.
- */
if (atomic_inc_return(&tgt->reqs) > 1)
vq = tgt->req_vq;
else {
--
1.7.9.5