From f259d9bdd2fb13926dd16ffacd3417d610c23e8c Mon Sep 17 00:00:00 2001 From: Ming Lei Date: Thu, 8 May 2014 15:23:45 +0800 Subject: 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 Acked-by: Paolo Bonzini Signed-off-by: Christoph Hellwig --- drivers/scsi/virtio_scsi.c | 53 ++++++---------------------------------------- 1 file changed, 6 insertions(+), 47 deletions(-) (limited to 'drivers/scsi/virtio_scsi.c') 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)) -- cgit v1.2.3-59-g8ed1b