From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CA7B2C54FCB for ; Thu, 23 Apr 2020 23:11:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B394A20780 for ; Thu, 23 Apr 2020 23:11:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729204AbgDWXLB (ORCPT ); Thu, 23 Apr 2020 19:11:01 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:50292 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728588AbgDWXGw (ORCPT ); Thu, 23 Apr 2020 19:06:52 -0400 Received: from [192.168.4.242] (helo=deadeye) by shadbolt.decadent.org.uk with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1jRkve-0004rl-1a; Fri, 24 Apr 2020 00:06:46 +0100 Received: from ben by deadeye with local (Exim 4.93) (envelope-from ) id 1jRkva-00E70M-13; Fri, 24 Apr 2020 00:06:42 +0100 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, Denis Kirjanov , "Jens Axboe" , "Davidlohr Bueso" , "Davidlohr Bueso" Date: Fri, 24 Apr 2020 00:07:18 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) X-Patchwork-Hint: ignore Subject: [PATCH 3.16 211/245] blktrace: re-write setting q->blk_trace In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.242 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.83-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Davidlohr Bueso commit cdea01b2bf98affb7e9c44530108a4a28535eee8 upstream. 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. (iii) Gets rid of the hacky-by-nature (void)xchg pattern. Signed-off-by: Davidlohr Bueso eviewed-by: Jeff Moyer Signed-off-by: Jens Axboe Signed-off-by: Ben Hutchings --- kernel/trace/blktrace.c | 16 +++++----------- 1 file changed, 5 insertions(+), 11 deletions(-) --- a/kernel/trace/blktrace.c +++ b/kernel/trace/blktrace.c @@ -448,7 +448,7 @@ int do_blk_trace_setup(struct request_qu 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, i; @@ -532,11 +532,8 @@ int do_blk_trace_setup(struct request_qu 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(); @@ -1550,7 +1547,7 @@ static int blk_trace_remove_queue(struct 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); @@ -1566,12 +1563,9 @@ static int blk_trace_setup_queue(struct 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; + ret = -EBUSY; + if (cmpxchg(&q->blk_trace, NULL, bt)) goto free_bt; - } if (atomic_inc_return(&blk_probes_ref) == 1) blk_register_tracepoints();