On 2020-08-17 03:08, Ming Lei wrote:
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 7c6dd6f75190..a62c29058d26 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -551,8 +551,27 @@ static void scsi_run_queue_async(struct scsi_device *sdev)
> if (scsi_target(sdev)->single_lun ||
> !list_empty(&sdev->host->starved_list))
> kblockd_schedule_work(&sdev->requeue_work);
> - else
> - blk_mq_run_hw_queues(sdev->request_queue, true);
> + else {
Has this patch been verified with checkpatch? Checkpatch should have warned
about the unbalanced braces.
> + /*
> + * smp_mb() implied in either rq->end_io or blk_mq_free_request
> + * is for ordering writing .device_busy in scsi_device_unbusy()
> + * and reading sdev->restarts.
> + */
Hmm ... I don't see what orders the atomic_dec(&sdev->device_busy) from
scsi_device_unbusy() and the atomic_read() below? I don't think that the block
layer guarantees ordering of these two memory accesses since both accesses
happen in the request completion path.
> + int old = atomic_read(&sdev->restarts);
> +
> + if (old) {
> + /*
> + * ->restarts has to be kept as non-zero if there is
> + * new budget contention comes.
There are two verbs in the above sentence ("is" and "comes"). Please remove
"comes" such that the sentence becomes grammatically correct.
> + *
> + * No need to run queue when either another re-run
> + * queue wins in updating ->restarts or one new budget
> + * contention comes.
> + */
> + if (atomic_cmpxchg(&sdev->restarts, old, 0) == old)
> + blk_mq_run_hw_queues(sdev->request_queue, true);
> + }
> + }
Please combine the two if-statements into a single if-statement using "&&"
to keep the indentation level low.
> @@ -1611,8 +1630,34 @@ static void scsi_mq_put_budget(struct request_queue *q)
> static bool scsi_mq_get_budget(struct request_queue *q)
> {
> struct scsi_device *sdev = q->queuedata;
> + int ret = scsi_dev_queue_ready(q, sdev);
> +
> + if (ret)
> + return true;
> +
> + atomic_inc(&sdev->restarts);
>
> - return scsi_dev_queue_ready(q, sdev);
> + /*
> + * Order writing .restarts and reading .device_busy, and make sure
> + * .restarts is visible to scsi_end_request(). Its pair is implied by
> + * __blk_mq_end_request() in scsi_end_request() for ordering
> + * writing .device_busy in scsi_device_unbusy() and reading .restarts.
> + *
> + */
> + smp_mb__after_atomic();
Barriers do not guarantee "is visible to". Barriers enforce ordering of memory
accesses performed by a certain CPU core. Did you perhaps mean that
sdev->restarts must be incremented before the code below reads sdev->device busy?
> + /*
> + * If all in-flight requests originated from this LUN are completed
> + * before setting .restarts, sdev->device_busy will be observed as
> + * zero, then blk_mq_delay_run_hw_queues() will dispatch this request
> + * soon. Otherwise, completion of one of these request will observe
> + * the .restarts flag, and the request queue will be run for handling
> + * this request, see scsi_end_request().
> + */
> + if (unlikely(atomic_read(&sdev->device_busy) == 0 &&
> + !scsi_device_blocked(sdev)))
> + blk_mq_delay_run_hw_queues(sdev->request_queue, SCSI_QUEUE_DELAY);
> + return false;
> }
Thanks,
Bart.
