Il 08/05/2014 09:23, Ming Lei ha scritto:
Access to tgt->req_vq is strictly serialized by spin_lock
of tgt->tgt_lock, so the ACCESS_ONCE() isn't necessary.
smp_read_barrier_depends() 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@xxxxxxxxxx>
Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
---
v2:
- take Paolo's comment on decrements of reqs
v1:
- fix comment on decrements of reqs with writing of req_vq
as suggested by Paolo
drivers/scsi/virtio_scsi.c | 53 +++++---------------------------------------
1 file changed, 6 insertions(+), 47 deletions(-)
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 16bfd50..13dd500 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -73,17 +73,12 @@ 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:
+ * tgt_lock is held to serialize reading and writing req_vq. Reading req_vq
+ * could be done locklessly, but we do not do it yet.
*
- * - 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.
+ * Decrements of reqs are never concurrent with writes of req_vq: before the
+ * decrement reqs will be != 0; after the decrement the virtqueue completion
+ * routine will not use the req_vq so it can be changed by a new request.
* Thus they can happen outside the tgt_lock, provided of course we make reqs
* an atomic_t.
*/
@@ -238,38 +233,6 @@ 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);
};
@@ -560,12 +523,8 @@ 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 = ACCESS_ONCE(tgt->req_vq);
+ vq = tgt->req_vq;
else {
queue_num = smp_processor_id();
while (unlikely(queue_num >= vscsi->num_queues))
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
--
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