From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752770AbbJ0DF4 (ORCPT ); Mon, 26 Oct 2015 23:05:56 -0400 Received: from mx2.suse.de ([195.135.220.15]:58478 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751594AbbJ0DFz (ORCPT ); Mon, 26 Oct 2015 23:05:55 -0400 Date: Mon, 26 Oct 2015 20:05:45 -0700 From: Davidlohr Bueso To: Jens Axboe Cc: Steven Rostedt , Ingo Molnar , linux-kernel@vger.kernel.org, dave@stgolabs.net Subject: [PATCH] blktrace: re-write setting q->blk_trace Message-ID: <20151027030545.GH27292@linux-uzut.site> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is really about simplifying the double xchg patterns into a single cmpxchg, with the same logic. Other than the immediate cleanup, there are some subtleties this change deals with: (i) While the load of the old bt is fully ordered wrt everything, ie: old_bt = xchg(&q->blk_trace, bt); [barrier] if (old_bt) (void) xchg(&q->blk_trace, old_bt); [barrier] blk_trace could still be changed between the xchg and the old_bt load. Note that this description is merely theoretical and afaict very small, but doing everything in a single context with cmpxchg closes this potential race. (ii) Ordering guarantees are obviously kept with cmpxchg. Signed-off-by: Davidlohr Bueso --- kernel/trace/blktrace.c | 15 ++++----------- 1 file changed, 4 insertions(+), 11 deletions(-) diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c index 90e72a0..4a30229 100644 --- a/kernel/trace/blktrace.c +++ b/kernel/trace/blktrace.c @@ -437,7 +437,7 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, struct block_device *bdev, struct blk_user_trace_setup *buts) { - struct blk_trace *old_bt, *bt = NULL; + struct blk_trace *bt = NULL; struct dentry *dir = NULL; int ret; @@ -519,11 +519,8 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, bt->trace_state = Blktrace_setup; ret = -EBUSY; - old_bt = xchg(&q->blk_trace, bt); - if (old_bt) { - (void) xchg(&q->blk_trace, old_bt); + if (cmpxchg(&q->blk_trace, NULL, bt)) goto err; - } if (atomic_inc_return(&blk_probes_ref) == 1) blk_register_tracepoints(); @@ -1481,7 +1478,7 @@ static int blk_trace_remove_queue(struct request_queue *q) static int blk_trace_setup_queue(struct request_queue *q, struct block_device *bdev) { - struct blk_trace *old_bt, *bt = NULL; + struct blk_trace *bt = NULL; int ret = -ENOMEM; bt = kzalloc(sizeof(*bt), GFP_KERNEL); @@ -1497,12 +1494,8 @@ static int blk_trace_setup_queue(struct request_queue *q, blk_trace_setup_lba(bt, bdev); - old_bt = xchg(&q->blk_trace, bt); - if (old_bt != NULL) { - (void)xchg(&q->blk_trace, old_bt); - ret = -EBUSY; + if (cmpxchg(&q->blk_trace, NULL, bt)) goto free_bt; - } if (atomic_inc_return(&blk_probes_ref) == 1) blk_register_tracepoints(); -- 2.1.4