linux-kernel - Re: [PATCH RFC] block, bfq: switch 'bfqg->ref' to use atomic refcount apis

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Date:   Mon, 2 Jan 2023 12:22:58 +0100
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     jack@...e.cz, tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk,
        paolo.valente@...aro.org, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC] block, bfq: switch 'bfqg->ref' to use atomic
 refcount apis

On Tue 27-12-22 14:09:40, Yu Kuai wrote:
> Hi, Jan!
> 
> 在 2022/12/27 11:15, Yu Kuai 写道:
> > From: Yu Kuai <yukuai3@...wei.com>
> > 
> > The updating of 'bfqg->ref' should be protected by 'bfqd->lock', however,
> > during code review, we found that bfq_pd_free() update 'bfqg->ref'
> > without holding the lock, which is problematic:
> > 
> > 1) bfq_pd_free() triggered by removing cgroup is called asynchronously;
> > 2) bfqq will grab bfqg reference, and exit bfqq will drop the reference,
> > which can concurrenty with 1).
> > 
> > Unfortunately, 'bfqd->lock' can't be held here because 'bfqd' might already
> > be freed in bfq_pd_free(). Fix the problem by using atomic refcount apis.
> > 
> > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > ---
> >   block/bfq-cgroup.c  | 8 +++-----
> >   block/bfq-iosched.h | 2 +-
> >   2 files changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> > index 1b2829e99dad..aa9c4f02e3a3 100644
> > --- a/block/bfq-cgroup.c
> > +++ b/block/bfq-cgroup.c
> > @@ -316,14 +316,12 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
> >   static void bfqg_get(struct bfq_group *bfqg)
> >   {
> > -	bfqg->ref++;
> > +	refcount_inc(&bfqg->ref);
> >   }
> >   static void bfqg_put(struct bfq_group *bfqg)
> >   {
> > -	bfqg->ref--;
> > -
> > -	if (bfqg->ref == 0)
> > +	if (refcount_dec_and_test(bfqg->ref))
> Sorry that here should be '&bfqg->ref'.
> 
> Anyway, I'll wait for you, and send a new version if you think this
> patch make sense.

Yes, the patch makes sense to me so feel free to send fixed version.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR