Re: Re: [PATCH 4/4] block: fix fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests to hctx->dispatch

[Muchun Song]

On Fri, Aug 23, 2024 at 7:28 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> On Sun, Aug 11, 2024 at 06:19:21 PM +0800, Muchun Song wrote:
> > Supposing the following scenario.
> >
> > CPU0                                                                CPU1
> >
> > blk_mq_request_issue_directly()                                     blk_mq_unquiesce_queue()
> >     if (blk_queue_quiesced())                                           blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)   3) store
> >         blk_mq_insert_request()                                         blk_mq_run_hw_queues()
> >             /*                                                              blk_mq_run_hw_queue()
> >              * Add request to dispatch list or set bitmap of                    if (!blk_mq_hctx_has_pending())     4) load
> >              * software queue.                  1) store                            return
> >              */
> >         blk_mq_run_hw_queue()
> >             if (blk_queue_quiesced())           2) load
> >                 return
> >             blk_mq_sched_dispatch_requests()
> >
> > The full memory barrier should be inserted between 1) and 2), as well as
> > between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
> > cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
> > Otherwise, either CPU will not re-run the hardware queue causing starvation.
>
> Memory barrier shouldn't serve as bug fix for two slow code paths.
>
> One simple fix is to add helper of blk_queue_quiesced_lock(), and
> call the following check on CPU0:
>
>         if (blk_queue_quiesced_lock())
>          blk_mq_run_hw_queue();

This only fixes blk_mq_request_issue_directly(), I think anywhere that
matching this
pattern (inserting a request to dispatch list and then running the
hardware queue)
should be fixed. And I think there are many places which match this
pattern (E.g.
blk_mq_submit_bio()). The above graph should be adjusted to the following.

CPU0                                        CPU1

blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
blk_mq_run_hw_queue()
blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
    if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
        return                                      blk_mq_run_hw_queue()
    blk_mq_sched_dispatch_requests()                    if
(!blk_mq_hctx_has_pending())     4) load
                                                            return

So I think fixing blk_mq_run_hw_queue() could cover all of the situations.
Maybe I thought wrongly. Please correct me.

Muchun,
Thanks.