Re: [PATCH v1] virtio_scsi: remove ACCESS_ONCE() and smp_read_barrier_depends()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/05/2014 07:24, 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@redhat.com>
> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> ---
> v1:
> 	- fix comment on decrements of reqs with writing of req_vq
> 	as suggested by Paolo
>
>  drivers/scsi/virtio_scsi.c |   55 +++++---------------------------------------
>  1 file changed, 6 insertions(+), 49 deletions(-)
>
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 16bfd50..bedea60 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -73,19 +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.
> - * Thus they can happen outside the tgt_lock, provided of course we make reqs
> - * an atomic_t.
> + * Decrements of reqs are never concurrent with writes of req_vq because
> + * request is always completd after being queued. Thus they can happen
> + * outside the tgt_lock, provided of course we make reqs an atomic_t.

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.

Otherwise looks good.

Paolo

>   */
>  struct virtio_scsi_target_state {
>  	/* This spinlock never held at the same time as vq_lock. */
> @@ -238,38 +231,6 @@ 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 +521,8 @@ 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))
>