On 2020-05-13 05:21, Christoph Hellwig wrote:
> Use of the BLK_MQ_REQ_FORCE is pretty bogus here..
>
>> + if (rq->rq_flags & RQF_PREEMPT)
>> + flags |= BLK_MQ_REQ_PREEMPT;
>> + if (reserved)
>> + flags |= BLK_MQ_REQ_RESERVED;
>> + /*
>> + * Queue freezing might be in-progress, and wait freeze can't be
>> + * done now because we have request not completed yet, so mark this
>> + * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
>> + * freeze hung forever.
>> + */
>> + flags |= BLK_MQ_REQ_FORCE;
>> +
>> + /* avoid allocation failure by clearing NOWAIT */
>> + nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
>> + if (!nrq)
>> + return;
>
> blk_get_request returns an ERR_PTR.
>
> But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
> just open code it and document what is going on:
>
> static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
> {
> struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>
> if (rq->q->elevator)
> return hctx->sched_tags;
> return hctx->tags;
> }
>
> static void blk_mq_resubmit_rq(struct request *rq)
> {
> struct blk_mq_alloc_data alloc_data = {
> .cmd_flags = rq->cmd_flags & ~REQ_NOWAIT;
> };
> struct request *nrq;
>
> if (rq->rq_flags & RQF_PREEMPT)
> alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
> if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
> alloc_data.flags |= BLK_MQ_REQ_RESERVED;
>
> /*
> * We must still be able to finish a resubmission due to a hotplug
> * even even if a queue freeze is in progress.
> */
> percpu_ref_get(&q->q_usage_counter);
> nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
> blk_queue_exit(q);
>
> if (!nrq)
> return; // XXX: warn?
> if (nrq->q->mq_ops->initialize_rq_fn)
> rq->mq_ops->initialize_rq_fn(nrq);
>
> blk_rq_copy_request(nrq, rq);
> ...
I don't like this because the above code allows allocation of requests
and tags while a request queue is frozen. I'm concerned that this will
break code that assumes that no tags are allocated while a request queue
is frozen. If a request queue has a single hardware queue with 64 tags,
if the above code allocates tag 40 and if blk_mq_tag_update_depth()
reduces the queue depth to 32, will nrq become a dangling pointer?
Thanks,
Bart.
