From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Return-Path: Sender: Tejun Heo Date: Wed, 11 Apr 2018 10:00:18 -0700 From: "tj@kernel.org" To: Bart Van Assche Cc: "00moses.alexander00@gmail.com" <00moses.alexander00@gmail.com>, "joseph.qi@linux.alibaba.com" , "nborisov@suse.com" , "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" , "gregkh@linuxfoundation.org" , "arnd@arndb.de" , "axboe@kernel.dk" , "shli@fb.com" Subject: Re: [PATCH v2] blk-cgroup: remove entries in blkg_tree before queue release Message-ID: <20180411170018.GL793541@devbig577.frc2.facebook.com> References: <20180407102148.GA9729@gmail.com> <20180409220938.GI3126663@devbig577.frc2.facebook.com> <20180411101242.GA2322@gmail.com> <20180411142019.GG793541@devbig577.frc2.facebook.com> <20180411142859.GB2322@gmail.com> <20180411144616.GI793541@devbig577.frc2.facebook.com> <20180411145123.GJ793541@devbig577.frc2.facebook.com> <20180411145632.GK793541@devbig577.frc2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: List-ID: Hello, On Wed, Apr 11, 2018 at 04:42:55PM +0000, Bart Van Assche wrote: > On Wed, 2018-04-11 at 07:56 -0700, Tejun Heo wrote: > > 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. > > I think that's a bad idea. A block driver is allowed to destroy the > spinlock it associated with the request queue as soon as blk_cleanup_queue() > has finished. If the block cgroup controller would cache a pointer to the > block driver spinlock then that could cause the cgroup code to attempt to > lock a spinlock after it has been destroyed. I don't think we need that kind > of race conditions. I see, but that problem is there with or without caching as long as we have queu_lock usage which reach beyond cleanup_queue, right? Whether that user caches the lock for matching unlocking or not doesn't really change the situation. Short of adding protection around queue_lock switching, I can't think of a solution tho. Probably the right thing to do is adding queue lock/unlock helpers which are safe to use beyond cleanup_queue. Thanks. -- tejun