linux-kernel - Re: [PATCH v2] blk-cgroup: remove entries in blkg

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Message-ID: <20180411145632.GK793541@devbig577.frc2.facebook.com>
Date:   Wed, 11 Apr 2018 07:56:32 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Alexandru Moise <00moses.alexander00@...il.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Bart Van Assche <bart.vanassche@....com>
Cc:     axboe@...nel.dk, shli@...com, nborisov@...e.com, arnd@...db.de,
        gregkh@...uxfoundation.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blk-cgroup: remove entries in blkg_tree before queue
 release

Hello, again.

On Wed, Apr 11, 2018 at 07:51:23AM -0700, Tejun Heo wrote:
> Oh, it wasn't Joseph's change.  It was Bart's fix for a problem
> reported by Joseph.  Bart, a063057d7c73 ("block: Fix a race between
> request queue removal and the block cgroup controller") created a
> regression where a request_queue can be destroyed with blkgs still
> attached.  The original report is..
> 
>  http://lkml.kernel.org/r/20180407102148.GA9729@gmail.com

And looking at the change, it looks like the right thing we should
have done is caching @lock on the print_blkg side and when switching
locks make sure both locks are held.  IOW, do the following in
blk_cleanup_queue()

	spin_lock_irq(lock);
	if (q->queue_lock != &q->__queue_lock) {
		spin_lock(&q->__queue_lock);
		q->queue_lock = &q->__queue_lock;
		spin_unlock(&q->__queue_lock);
	}
	spin_unlock_irq(lock);

Otherwise, there can be two lock holders thinking they have exclusive
access to the request_queue.

Thanks.

-- 
tejun