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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 E3867C32771 for ; Wed, 15 Jan 2020 16:42:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A3CC92467C for ; Wed, 15 Jan 2020 16:42:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579106531; bh=vjko50FmDt2UVx7s7dkC4h+TY7ecAco1ZYFz1nhr/Q0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=q0sVv/QS/0cvRl4Nw0/tfdYPeInlOVp5oGaghxOlpVlo6MEqn6p1Fa4typgfPQMOi AXUFtnw4VC+O8jFMPgiGDmA5MVWB+9kmTNjltmnpoIaZVh4EOWVkvuvWWwSxB4bUBf n46gyvc9w44QjYj32OEfhRSuyshgsGqr04JlWP50= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728913AbgAOQmL (ORCPT ); Wed, 15 Jan 2020 11:42:11 -0500 Received: from mail.kernel.org ([198.145.29.99]:34748 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726483AbgAOQmK (ORCPT ); Wed, 15 Jan 2020 11:42:10 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 470312081E; Wed, 15 Jan 2020 16:42:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579106529; bh=vjko50FmDt2UVx7s7dkC4h+TY7ecAco1ZYFz1nhr/Q0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=CrJcrB++nluNYKf/mwXWojkgdHPndUFdi30j3DWcEtQX+IjkjDKekXcJVeM3iZrot Wb1PfwS8oTXRhB751jRhOSLmfIQWUxetR5Hv/v8WZHbN5MQ7bnrTw3feD27iMCdvvC I0a5cTUF0382tLyvOBAoWVOuYRpxlrRrG63htL5c= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id F25083520BAE; Wed, 15 Jan 2020 08:42:08 -0800 (PST) Date: Wed, 15 Jan 2020 08:42:08 -0800 From: "Paul E. McKenney" To: Eric Dumazet Cc: Stephen Rothwell , Linux Next Mailing List , Linux Kernel Mailing List Subject: Re: linux-next: build warning after merge of the rcu tree Message-ID: <20200115164208.GA14832@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191212160622.021517d3@canb.auug.org.au> <20191212060200.GW2889@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-next-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-next@vger.kernel.org On Wed, Dec 11, 2019 at 10:57:24PM -0800, Eric Dumazet wrote: > On Wed, Dec 11, 2019 at 10:38 PM Eric Dumazet wrote: > > On Wed, Dec 11, 2019 at 10:02 PM Paul E. McKenney wrote: > > > On Thu, Dec 12, 2019 at 04:06:22PM +1100, Stephen Rothwell wrote: > > > > Hi all, > > > > > > > > After merging the rcu (I think) tree, today's linux-next build (x86_64 > > > > allnoconfig) produced this warning: > > > > > > > > kernel/time/timer.c: In function 'schedule_timeout': > > > > kernel/time/timer.c:969:20: warning: 'timer.expires' may be used uninitialized in this function [-Wmaybe-uninitialized] > > > > 969 | long diff = timer->expires - expires; > > > > | ~~~~~^~~~~~~~~ > > > > > > > > Introduced by (bisected to) commit > > > > > > > > c4127fce1d02 ("timer: Use hlist_unhashed_lockless() in timer_pending()") > > > > > > > > x86_64-linux-gnu-gcc (Debian 9.2.1-21) 9.2.1 20191130 > > > > > > Well, if the timer is pending, then ->expires has to have been > > > initialized, but off where the compiler cannot see it, such as during a > > > previous call to __mod_timer(). And the change may have made it harder > > > for the compiler to see all of these relationships, but... > > > > > > I don't see this warning with gcc version 7.4.0. Just out of curiosity, > > > what are you running, Stephen? > > > > > > Eric, any thoughts for properly educating the compiler on this one? > > > > Ah... the READ_ONCE() apparently turns off the compiler ability to > > infer that this branch should not be taken. > > > > Since __mod_timer() is inlined we could perhaps add a new option > > > > diff --git a/kernel/time/timer.c b/kernel/time/timer.c > > index 4820823515e9..8bbce552568b 100644 > > --- a/kernel/time/timer.c > > +++ b/kernel/time/timer.c > > @@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct > > timer_list *timer, > > > > #define MOD_TIMER_PENDING_ONLY 0x01 > > #define MOD_TIMER_REDUCE 0x02 > > +#define MOD_TIMER_NOTPENDING 0x04 > > > > static inline int > > __mod_timer(struct timer_list *timer, unsigned long expires, unsigned > > int options) > > @@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned > > long expires, unsigned int option > > * the timer is re-modified to have the same timeout or ends up in the > > * same array bucket then just return: > > */ > > - if (timer_pending(timer)) { > > + if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) { > > /* > > * The downside of this optimization is that it can result in > > * larger granularity than you would get from adding a new > > @@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout) > > > > timer.task = current; > > timer_setup_on_stack(&timer.timer, process_timeout, 0); > > - __mod_timer(&timer.timer, expire, 0); > > + __mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING); > > schedule(); > > del_singleshot_timer_sync(&timer.timer); > > > Also add_timer() can benefit from the same hint, since it seems inlined as well. > > (untested patch) Apologies for the delay, fat fingers and holidays... :-/ I folded this into your earlier patch, resulting in the patch at the end. Could you please let me know whether this matches your intent? Thanx, Paul > diff --git a/kernel/time/timer.c b/kernel/time/timer.c > index 4820823515e9..568564ae3597 100644 > --- a/kernel/time/timer.c > +++ b/kernel/time/timer.c > @@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct > timer_list *timer, > > #define MOD_TIMER_PENDING_ONLY 0x01 > #define MOD_TIMER_REDUCE 0x02 > +#define MOD_TIMER_NOTPENDING 0x04 > > static inline int > __mod_timer(struct timer_list *timer, unsigned long expires, unsigned > int options) > @@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned > long expires, unsigned int option > * the timer is re-modified to have the same timeout or ends up in the > * same array bucket then just return: > */ > - if (timer_pending(timer)) { > + if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) { > /* > * The downside of this optimization is that it can result in > * larger granularity than you would get from adding a new > @@ -1133,7 +1134,7 @@ EXPORT_SYMBOL(timer_reduce); > void add_timer(struct timer_list *timer) > { > BUG_ON(timer_pending(timer)); > - mod_timer(timer, timer->expires); > + __mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING); > } > EXPORT_SYMBOL(add_timer); > > @@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout) > > timer.task = current; > timer_setup_on_stack(&timer.timer, process_timeout, 0); > - __mod_timer(&timer.timer, expire, 0); > + __mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING); > schedule(); > del_singleshot_timer_sync(&timer.timer); > ------------------------------------------------------------------------ commit 704c46852c8f8c15cc0ecb45b19f8d3cd983faa6 Author: Eric Dumazet Date: Thu Nov 7 11:37:38 2019 -0800 timer: Use hlist_unhashed_lockless() in timer_pending() The timer_pending() function is mostly used in lockless contexts, so Without proper annotations, KCSAN might detect a data-race [1]. Using hlist_unhashed_lockless() instead of hand-coding it seems appropriate (as suggested by Paul E. McKenney). [1] BUG: KCSAN: data-race in del_timer / detach_if_pending write to 0xffff88808697d870 of 8 bytes by task 10 on cpu 0: __hlist_del include/linux/list.h:764 [inline] detach_timer kernel/time/timer.c:815 [inline] detach_if_pending+0xcd/0x2d0 kernel/time/timer.c:832 try_to_del_timer_sync+0x60/0xb0 kernel/time/timer.c:1226 del_timer_sync+0x6b/0xa0 kernel/time/timer.c:1365 schedule_timeout+0x2d2/0x6e0 kernel/time/timer.c:1896 rcu_gp_fqs_loop+0x37c/0x580 kernel/rcu/tree.c:1639 rcu_gp_kthread+0x143/0x230 kernel/rcu/tree.c:1799 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352 read to 0xffff88808697d870 of 8 bytes by task 12060 on cpu 1: del_timer+0x3b/0xb0 kernel/time/timer.c:1198 sk_stop_timer+0x25/0x60 net/core/sock.c:2845 inet_csk_clear_xmit_timers+0x69/0xa0 net/ipv4/inet_connection_sock.c:523 tcp_clear_xmit_timers include/net/tcp.h:606 [inline] tcp_v4_destroy_sock+0xa3/0x3f0 net/ipv4/tcp_ipv4.c:2096 inet_csk_destroy_sock+0xf4/0x250 net/ipv4/inet_connection_sock.c:836 tcp_close+0x6f3/0x970 net/ipv4/tcp.c:2497 inet_release+0x86/0x100 net/ipv4/af_inet.c:427 __sock_release+0x85/0x160 net/socket.c:590 sock_close+0x24/0x30 net/socket.c:1268 __fput+0x1e1/0x520 fs/file_table.c:280 ____fput+0x1f/0x30 fs/file_table.c:313 task_work_run+0xf6/0x130 kernel/task_work.c:113 tracehook_notify_resume include/linux/tracehook.h:188 [inline] exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163 Reported by Kernel Concurrency Sanitizer on: CPU: 1 PID: 12060 Comm: syz-executor.5 Not tainted 5.4.0-rc3+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, Signed-off-by: Eric Dumazet Cc: Thomas Gleixner [ paulmck: Pulled in Eric's later amendments. ] Signed-off-by: Paul E. McKenney diff --git a/include/linux/timer.h b/include/linux/timer.h index 1e6650e..0dc19a8 100644 --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -164,7 +164,7 @@ static inline void destroy_timer_on_stack(struct timer_list *timer) { } */ static inline int timer_pending(const struct timer_list * timer) { - return timer->entry.pprev != NULL; + return !hlist_unhashed_lockless(&timer->entry); } extern void add_timer_on(struct timer_list *timer, int cpu); diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 4820823..568564a 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct timer_list *timer, #define MOD_TIMER_PENDING_ONLY 0x01 #define MOD_TIMER_REDUCE 0x02 +#define MOD_TIMER_NOTPENDING 0x04 static inline int __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int options) @@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option * the timer is re-modified to have the same timeout or ends up in the * same array bucket then just return: */ - if (timer_pending(timer)) { + if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) { /* * The downside of this optimization is that it can result in * larger granularity than you would get from adding a new @@ -1133,7 +1134,7 @@ EXPORT_SYMBOL(timer_reduce); void add_timer(struct timer_list *timer) { BUG_ON(timer_pending(timer)); - mod_timer(timer, timer->expires); + __mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING); } EXPORT_SYMBOL(add_timer); @@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout) timer.task = current; timer_setup_on_stack(&timer.timer, process_timeout, 0); - __mod_timer(&timer.timer, expire, 0); + __mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING); schedule(); del_singleshot_timer_sync(&timer.timer);