linux-kernel - Re: [PATCH V2 1/2] blk-mq: init hctx sched after update ctx and hctx mapping

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Date:   Mon, 20 Aug 2018 12:18:39 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Ming Lei <tom.leiming@...il.com>, Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <bart.vanassche@....com>,
        Keith Busch <keith.busch@...ux.intel.com>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/2] blk-mq: init hctx sched after update ctx and hctx
 mapping

On Mon, Aug 20, 2018 at 11:57:52AM +0800, jianchao.wang wrote:
> Hi Ming
> 
> On 08/20/2018 10:24 AM, Ming Lei wrote:
> >> The code path is:
> >>
> >> elevator_release
> >>   -> elevator_put(e->type)
> >>     -> module_put(e->elevator_owner)
> >>
> >> In normal elevator switch path, elevator_get will hold a reference counter of the
> >> elevator_owner.
> >> In this patch set, the elevator_type is saved directly. To prevent the io scheduler module
> >> is removed, we need to hold a reference of the module.
> > Yeah, I agree that the module reference need to be held, but it need to be
> > released too.
> > 
> > My concern is that this introduced getting module reference in your patch
> > isn't released. The module reference is a counter too, so the get and
> > put operation should be matched.
> 
> elevator_switch_mq
>   -> elevator_exit
> it will put a reference count of the elevator_queue associated with the request_queue.
> and the elevator_release will be invoked when the reference count of elevator_queue->kobj reaches zero.
> elevator_release will put the reference count of the io scheduler module.
> 
> the elevator_queue structure will be allocated and freed every time when we switch io scheduler.
> so the elevator_release will always be invoked.
> 
> This is the put ref corresponding to the get one in this patch.

OK, got it, the added __module_get() is just like what elevator_get()
does from __elevator_change(). Sorry for the noise.

Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thanks,
Ming