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[157.230.128.187]) by smtp.gmail.com with ESMTPSA id o15sm12562445pgj.60.2020.04.15.05.34.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Apr 2020 05:34:35 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id 1649340277; Wed, 15 Apr 2020 12:34:34 +0000 (UTC) Date: Wed, 15 Apr 2020 12:34:34 +0000 From: Luis Chamberlain To: Christoph Hellwig Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org, 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: <20200415123434.GU11244@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200415071425.GA21099@infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 15, 2020 at 12:14:25AM -0700, Christoph Hellwig wrote: > On Wed, Apr 15, 2020 at 06:16:49AM +0000, Luis Chamberlain wrote: > > The BLKTRACESETUP above works on request_queue which later > > LOOP_CTL_DEL races on and sweeps the debugfs dir underneath us. > > If you use this commit alone though, this doesn't fix the race issue > > however, and that's because of both still the debugfs_lookup() use > > and that we're still using asynchronous removal at this point. > > > > refcounting will just ensure we don't take the request_queue underneath > > our noses. > > > > Should I just add this to the commit log? > > That sounds much more useful than the trace. > > 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. Think that's worthy review? Luis