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authorMing Lei <tom.leiming@gmail.com>2014-05-08 15:23:45 +0800
committerChristoph Hellwig <hch@lst.de>2014-05-19 19:12:26 +0200
commitf259d9bdd2fb13926dd16ffacd3417d610c23e8c (patch)
treebeb1cc910276e95e9e8623366fca0a26fb76a570 /drivers/scsi/virtio_scsi.c
parent27f344eb15dd0da80ebec80c7245e8c85043f841 (diff)
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virtio_scsi: remove ACCESS_ONCE() and smp_read_barrier_depends()
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. Signed-off-by: Ming Lei <tom.leiming@gmail.com> Acked-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Christoph Hellwig <hch@lst.de>
Diffstat (limited to 'drivers/scsi/virtio_scsi.c')
-rw-r--r--drivers/scsi/virtio_scsi.c53
1 files changed, 6 insertions, 47 deletions
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index db3b494e5926..fc054935eb1f 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))