virtio_scsi: remove ACCESS_ONCE() and smp_read_barrier_depends() (f259d9bd) · Commits · 戴 / test

drivers/scsi/virtio_scsi.c

+6 −47

Original line number	Diff line number	Diff line
		@@ -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))

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