Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get_req()

[yukuai (C)]

On 2021/09/08 17:27, Ming Lei wrote:
> On Wed, Sep 08, 2021 at 04:29:57PM +0800, yukuai (C) wrote:
> > On 2021/09/08 16:00, Ming Lei wrote:
> > > On Wed, Sep 08, 2021 at 03:37:06PM +0800, yukuai (C) wrote:
> > > > On 2021/09/08 15:30, Ming Lei wrote:
> > > >
> > > > > > +put_req:
> > > > > > +	if (req)
> > > > > > +		blk_mq_put_rq_ref(req);
> > > > > >     	return ret ? ERR_PTR(ret) : cmd;
> > > > >
> > > > > After the request's refcnt is dropped, it can be freed immediately, then
> > > > > one stale command is returned to caller.
> > > >
> > > > Hi, Ming
> > > >
> > > > It's right this patch leave this problem. Please take a look at patch 3
> > > > and patch 4, the problem should be fixed with these patches.
> > >
> > > Not see it is addressed in patch 3 & 4, and it is one obvious fault in
> > > patch 2, please fix it from beginning by moving the refcnt drop
> > > into recv_work().
> >
> > Hi, Ming
> >
> > With patch 3 & 4:
> >
> > if nbd_read_stat() return a valid cmd, then the refcnt should not drop
> > to 0 before blk_mq_complete_request() in recv_work().
>
> The valid cmd won't be timed out just between nbd_read_stat() and
> calling blk_mq_complete_request()?
>
> Yeah, the issue is addressed by patch 4, then please put 2 after
> 3 & 4, and suggest to add comment why request ref won't drop to zero
> in recv_work().

Hi, Ming

Thanks for the advice, will do this in next iteration.

Best regards
Kuai
> > if nbd_read_stat() failed, it won't be a problem if the request is freed
> > immediately when refcnt is dropped in nbd_read_stat().
> >
> > That's why I said that the problem will be fixed.
> >
> > BTW, if we move the refcnt drop into recv_work, we can only do that if
> > nbd_read_stat() return a valid cmd. If we get a valid rq and failed in
> > the following checkings in nbd_read_stat(), it's better to drop the
> > refcnt in nbd_read_stat().
>
> The usual pattern is to drop the refcnt on when the object isn't used
> any more, so it is perfectly fine to hold the ref until that time.
>
>
> Thanks,
> Ming
>
> .