Re: [PATCH] block: fix io hung by block throttle

Junxiao Bi
2021-04-17T21:39:06Z

On 4/17/21 3:10 AM, Hillf Danton wrote:

>> --- a/block/blk-rq-qos.c
>> +++ b/block/blk-rq-qos.c
>> @@ -260,19 +260,17 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
>> .cb = acquire_inflight_cb,
>> .private_data = private_data,
>> };
>> - bool has_sleeper;
>>
>> - has_sleeper = wq_has_sleeper(&rqw->wait);
>> - if (!has_sleeper && acquire_inflight_cb(rqw, private_data))
>> + if (!wq_has_sleeper(&rqw->wait)
>> + && acquire_inflight_cb(rqw, private_data))
>> return;
>>
>> prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
>> - has_sleeper = !wq_has_single_sleeper(&rqw->wait);
>> do {
>> /* The memory barrier in set_task_state saves us here. */
>> if (data.got_token)
>> break;
>> - if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) {
>> + if (acquire_inflight_cb(rqw, private_data)) {
>> finish_wait(&rqw->wait, &data.wq);
> Simply removing !has_sleeper is not enough if it is mandatory before
> acquire_inflight_cb() without adding something like a mutex to sieve the
> concurrent sleepers out, see below.
>
>
> --- a/block/blk-rq-qos.c
> +++ b/block/blk-rq-qos.c
> @@ -260,19 +260,18 @@ void rq_qos_wait(struct rq_wait *rqw, vo
> .cb = acquire_inflight_cb,
> .private_data = private_data,
> };
> - bool has_sleeper;
>
> - has_sleeper = wq_has_sleeper(&rqw->wait);
> - if (!has_sleeper && acquire_inflight_cb(rqw, private_data))
> - return;
> + mutex_lock(&rqw->mutex);
> +
> + if (acquire_inflight_cb(rqw, private_data))

This function is to increase atomic variable rq_wait->inflight. What's
the mutex for?

Thanks,

Junxiao.

> + goto out;
>
> prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
> - has_sleeper = !wq_has_single_sleeper(&rqw->wait);
> do {
> /* The memory barrier in set_task_state saves us here. */
> if (data.got_token)
> break;
> - if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) {
> + if (acquire_inflight_cb(rqw, private_data)) {
> finish_wait(&rqw->wait, &data.wq);
>
> /*
> @@ -286,10 +285,11 @@ void rq_qos_wait(struct rq_wait *rqw, vo
> break;
> }
> io_schedule();
> - has_sleeper = true;
> set_current_state(TASK_UNINTERRUPTIBLE);
> } while (1);
> finish_wait(&rqw->wait, &data.wq);
> +out:
> + mutex_unlock(&rqw->mutex);
> }