Re: [PATCH] blk-mq: put the reference of the io scheduler module after switching back

[Yu Kuai]

在 2022/10/13 21:47, Jinlong Chen 写道:
> Hi, Yu Kuai!
>
> > I'm confused here, cause I do think this patch make sense.
> >
> > blk_mq_update_nr_hw_queues
> >    // for each queue using the tagset
> >    blk_mq_freeze_queue
> >    // if current elevator is not none, swith to none
> >    blk_mq_elv_switch_none
> >     // elevator need to be switched back, got a reference to
> >     // prevent module to be removed.
> >     __module_get
> >     elevator_switch(q, NULL);
> >
> >    // switch back from none elevator
> >    blk_mq_elv_switch_back
> >     -> should release the module reference here
> >    blk_mq_unfreeze_queue
>
> We need to release the reference only if blk_mq_elv_switch_back got its own
>   reference. However, blk_mq_elv_switch_back (precisely elevator_switch_mq)
>   does not increase the reference of the module it is switching to.
>   Hence, the reference got in blk_mq_elv_switch_none does not need to be released,
>   or we'll have to re-increase the reference count manually.'

But I don't see elevator_switch() release the referenct of the module
it is switching from. It's still not balance to me.

Thanks,
Kuai
> > By the way, I do not test yet, but I think problem can be reporduced:
> >
> >
> > 1. modprobe bfq
> > 2. switch elevator to bfq
> > 3. trigger blk_mq_update_nr_hw_queues
> > 4. switch elevator to none
> > 5. rmmod bfq will fail
>
> I tried to reproduce the problem but failed, so I found my mistake.
>
> Sincerely,
> Jinlong Chen
> .