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[157.230.128.187]) by smtp.gmail.com with ESMTPSA id s74sm150560pgc.50.2020.04.15.18.12.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Apr 2020 18:12:48 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id E7A8440277; Thu, 16 Apr 2020 01:12:47 +0000 (UTC) Date: Thu, 16 Apr 2020 01:12:47 +0000 From: Luis Chamberlain To: Bart Van Assche Cc: Christoph Hellwig , axboe@kernel.dk, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Omar Sandoval , Hannes Reinecke , Michal Hocko Subject: Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl Message-ID: <20200416011247.GB11244@42.do-not-panic.com> References: <20200414041902.16769-1-mcgrof@kernel.org> <20200414041902.16769-4-mcgrof@kernel.org> <20200414154044.GB25765@infradead.org> <20200415061649.GS11244@42.do-not-panic.com> <20200415071425.GA21099@infradead.org> <20200415123434.GU11244@42.do-not-panic.com> <73332d32-b095-507f-fb2a-68460533eeb7@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <73332d32-b095-507f-fb2a-68460533eeb7@acm.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Apr 15, 2020 at 07:18:22AM -0700, Bart Van Assche wrote: > On 2020-04-15 05:34, Luis Chamberlain wrote: > > On Wed, Apr 15, 2020 at 12:14:25AM -0700, Christoph Hellwig wrote: > >> Btw, Isn't blk_get_queue racy as well? Shouldn't we check > >> blk_queue_dying after getting the reference and undo it if the queue is > >> indeeed dying? > > > > Yes that race should be possible: > > > > bool blk_get_queue(struct request_queue *q) > > { > > if (likely(!blk_queue_dying(q))) { > > ----------> we can get the queue to go dying here <--------- > > __blk_get_queue(q); > > return true; > > } > > > > return false; > > } > > EXPORT_SYMBOL(blk_get_queue); > > > > I'll pile up a fix. I've also considered doing a full review of callers > > outside of the core block layer using it, and maybe just unexporting > > this. It was originally exported due to commit d86e0e83b ("block: export > > blk_{get,put}_queue()") to fix a scsi bug, but I can't find such > > respective fix. I suspec that using bdgrab()/bdput() seems more likely > > what drivers should be using. That would allow us to keep this > > functionality internal. > > blk_get_queue() prevents concurrent freeing of struct request_queue but > does not prevent concurrent blk_cleanup_queue() calls. Wouldn't concurrent blk_cleanup_queue() calls be a bug? If so should I make it clear that it would be or simply prevent it? > Callers of > blk_get_queue() may encounter a change of the queue state from normal > into dying any time during the blk_get_queue() call or after > blk_get_queue() has finished. Maybe I'm overlooking something but I > doubt that modifying blk_get_queue() will help. Good point, to fix that race described by Christoph we'd have to take into consideration refcounts of the request_queue to prevent queues from changing state to dying if the refcount is > 1, however that'd also would mean not allowing the request_queue from ever dying. One way we could resolve this could be to to keep track of a quiesce/dying request, then at that point prevent blk_get_queue() from allowing increments, and once the refcount is down to 1, flip the switch to dying. Luis