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Date:   Wed, 26 Apr 2023 10:20:34 +0800
From:   Tao Su <tao1.su@linux.intel.com>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        tj@kernel.org, josef@toxicpanda.com, axboe@kernel.dk,
        "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH] Remove blkg node after destroying blkg
Message-ID: <ZEiKcqqxqWtY2MQA@linux.bj.intel.com>
References: <20230425075911.839539-1-tao1.su@linux.intel.com>
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On Wed, Apr 26, 2023 at 09:13:08AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2023/04/25 19:09, Yu Kuai 写道:
> > Hi,
> > 
> > 在 2023/04/25 17:41, Tao Su 写道:
> > > On Tue, Apr 25, 2023 at 04:09:34PM +0800, Yu Kuai wrote:
> > > > Hi,
> > > > 
> > > > 在 2023/04/25 15:59, Tao Su 写道:
> > > > > Kernel hang when poweroff or reboot, due to infinite restart
> > > > > in function
> > > > > blkg_destroy_all. It will goto restart label when a batch of blkgs are
> > > > > destroyed, but not remove blkg node in blkg_list. So the blkg_list is
> > > > > same in every 'restart' and result in kernel hang.
> > > > > 
> > > > > By adding list_del to remove blkg node after destroying, can solve this
> > > > > kernel hang issue and satisfy the previous will to 'restart'.
> > > > > 
> > > > > Reported-by: Xiangfei Ma <xiangfeix.ma@intel.com>
> > > > > Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
> > > > > Tested-by: Farrah Chen <farrah.chen@intel.com>
> > > > > Signed-off-by: Tao Su <tao1.su@linux.intel.com>
> > > > > ---
> > > > >    block/blk-cgroup.c | 1 +
> > > > >    1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> > > > > index bd50b55bdb61..960eb538a704 100644
> > > > > --- a/block/blk-cgroup.c
> > > > > +++ b/block/blk-cgroup.c
> > > > > @@ -530,6 +530,7 @@ static void blkg_destroy_all(struct gendisk *disk)
> > > > >            spin_lock(&blkcg->lock);
> > > > >            blkg_destroy(blkg);
> > > > > +        list_del(&blkg->q_node);
> > > > 
> > > > blkg should stay on the queue list until blkg_free_workfn(), otherwise
> > > > parent blkg can be freed before child, which will cause some known
> > > > issue.
> > > 
> > > Yes, directly removing blkg node is not appropriate, which I noticed some
> > > comments in blkg_destroy(), thanks for pointing out this issue.
> > > 
> > > > 
> > > > I think this hung happens when total blkg is greater than
> > > > BLKG_DESTROY_BATCH_SIZE, right?
> > > 
> > > Yes, you are right.
> > > 
> > > > 
> > > > Can you try if following patch fix your problem?
> > > 
> > > This patch can also fix my problem, and indeed is a more secure way.
> > 
> > Thanks for the test, for a better solution, I think 'blkcg_mutex' can
> > be used to protect 'blkg->q_node' list instead of 'queue_lock', so that
> > the 'restart' can be removed because softlockup can be avoided.
> > 
> 
> I looked into this, and I found that this is not a easy thing to do.
> 
> Anyway, feel free to submit a new patch based on my orignial suggestion.

Thanks for your contribution and careful review, I will submit the new
patch if no other comments.

Thanks,
Tao

> 
> Thanks,
> Kuai
>