linux-kernel - Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get

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Message-ID: <YTiB8TLQ7ZJWvS7Y@T590>
Date:   Wed, 8 Sep 2021 17:27:13 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     axboe@...nel.dk, josef@...icpanda.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, nbd@...er.debian.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get_req()

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().

> 
> 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