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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>
This commit is contained in:
Ming Lei 2014-05-08 15:23:45 +08:00 committed by Christoph Hellwig
parent 27f344eb15
commit f259d9bdd2
1 changed files with 6 additions and 47 deletions
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53
drivers/scsi/virtio_scsi.c
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@ -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))