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Date:   Wed, 14 Sep 2022 11:00:36 +0200
From:   Jan Kara <jack@suse.cz>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Paolo VALENTE <paolo.valente@unimore.it>, Jan Kara <jack@suse.cz>,
        cgroups@vger.kernel.org, linux-block <linux-block@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        LKML <linux-kernel@vger.kernel.org>, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH -next v10 3/4] block, bfq: refactor the counting of
 'num_groups_with_pending_reqs'
Message-ID: <20220914090036.46zsrj2l23ubvvk6@quack3>
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Hi guys!

On Wed 14-09-22 16:15:26, Yu Kuai wrote:
> 在 2022/09/14 15:50, Paolo VALENTE 写道:
> > 
> > 
> > > Il giorno 14 set 2022, alle ore 03:55, Yu Kuai <yukuai1@huaweicloud.com> ha scritto:
> > > 
> > > 
> > > 
> > > 在 2022/09/07 9:16, Yu Kuai 写道:
> > > > Hi, Paolo!
> > > > 在 2022/09/06 17:37, Paolo Valente 写道:
> > > > > 
> > > > > 
> > > > > > Il giorno 26 ago 2022, alle ore 04:34, Yu Kuai <yukuai1@huaweicloud.com> ha scritto:
> > > > > > 
> > > > > > Hi, Paolo!
> > > > > > 
> > > > > > 在 2022/08/25 22:59, Paolo Valente 写道:
> > > > > > > > Il giorno 11 ago 2022, alle ore 03:19, Yu Kuai <yukuai1@huaweicloud.com <mailto:yukuai1@huaweicloud.com>> ha scritto:
> > > > > > > > 
> > > > > > > > Hi, Paolo
> > > > > > > > 
> > > > > > > > 在 2022/08/10 18:49, Paolo Valente 写道:
> > > > > > > > > > Il giorno 27 lug 2022, alle ore 14:11, Yu Kuai <yukuai1@huaweicloud.com <mailto:yukuai1@huaweicloud.com>> ha scritto:
> > > > > > > > > > 
> > > > > > > > > > Hi, Paolo
> > > > > > > > > > 
> > > > > > > > > hi
> > > > > > > > > > Are you still interested in this patchset?
> > > > > > > > > > 
> > > > > > > > > Yes. Sorry for replying very late again.
> > > > > > > > > Probably the last fix that you suggest is enough, but I'm a little bit
> > > > > > > > > concerned that it may be a little hasty.  In fact, before this fix, we
> > > > > > > > > exchanged several messages, and I didn't seem to be very good at
> > > > > > > > > convincing you about the need to keep into account also in-service
> > > > > > > > > I/O.  So, my question is: are you sure that now you have a
> > > > > > > > 
> > > > > > > > I'm confused here, I'm pretty aware that in-service I/O(as said pending
> > > > > > > > requests is the patchset) should be counted, as you suggested in v7, are
> > > > > > > > you still thinking that the way in this patchset is problematic?
> > > > > > > > 
> > > > > > > > I'll try to explain again that how to track is bfqq has pending pending
> > > > > > > > requests, please let me know if you still think there are some problems:
> > > > > > > > 
> > > > > > > > patch 1 support to track if bfqq has pending requests, it's
> > > > > > > > done by setting the flag 'entity->in_groups_with_pending_reqs' when the
> > > > > > > > first request is inserted to bfqq, and it's cleared when the last
> > > > > > > > request is completed. specifically the flag is set in
> > > > > > > > bfq_add_bfqq_busy() when 'bfqq->dispatched' if false, and it's cleared
> > > > > > > > both in bfq_completed_request() and bfq_del_bfqq_busy() when
> > > > > > > > 'bfqq->diapatched' is false.
> > > > > > > > 
> > > > > > > This general description seems correct to me. Have you already sent a new version of your patchset?
> > > > > > 
> > > > > > It's glad that we finially on the same page here.
> > > > > > 
> > > > > 
> > > > > Yep. Sorry for my chronicle delay.
> > > > Better late than never ????
> > > > > 
> > > > > > Please take a look at patch 1, which already impelement the above
> > > > > > descriptions, it seems to me there is no need to send a new version
> > > > > > for now. If you think there are still some other problems, please let
> > > > > > me know.
> > > > > > 
> > > > > 
> > > > > Patch 1 seems ok to me. I seem to have only one pending comment on this patch (3/4) instead. Let me paste previous stuff here for your convenience:
> > > > That sounds good.
> > > > > 
> > > > > > > 
> > > > > > > -    /*
> > > > > > > -     * Next function is invoked last, because it causes bfqq to be
> > > > > > > -     * freed if the following holds: bfqq is not in service and
> > > > > > > -     * has no dispatched request. DO NOT use bfqq after the next
> > > > > > > -     * function invocation.
> > > > > > > -     */
> > > > > > I would really love it if you leave this comment.  I added it after
> > > > > > suffering a lot for a nasty UAF.  Of course the first sentence may
> > > > > > need to be adjusted if the code that precedes it is to be removed.
> > > > > > Same as above, if this patch is applied, this function will be gone.
> > > 
> > > Hi, I'm curious while I'm trying to add the comment, before this
> > > patchset, can bfqq be freed when bfq_weights_tree_remove is called?
> > > 
> > > bfq_completed_request
> > > bfqq->dispatched--
> > > if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq))
> > >   bfq_weights_tree_remove(bfqd, bfqq);
> > > 
> > > // continue to use bfqq
> > > 
> > > It seems to me this is problematic if so, because bfqq is used after
> > > bfq_weights_tree_remove() is called.
> > > 
> > 
> > It is.  Yet, IIRC, I verified that bfqq was not used after that free,
> > and I added that comment as a heads-up.  What is a scenario (before
> > your pending modifications) where this use-after-free happens?
> > 
> 
> No, it never happens, I just notice it because it'll be weird if I
> place the comment where bfq_weights_tree_remove() is called, since bfqq
> will still be accessed.
> 
> If the suituation that the comment says is possible, perhaps we should
> move bfq_weights_tree_remove() to the last of bfq_completed_request().
> However, it seems that we haven't meet the problem for quite a long
> time...

I'm bit confused which comment you are speaking about but
bfq_completed_request() gets called only from bfq_finish_requeue_request()
and the request itself still holds a reference to bfqq. Only later in
bfq_finish_requeue_request() when we do:

	bfqq_request_freed(bfqq);
	bfq_put_queue(bfqq);

bfqq can get freed.

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