From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751966Ab2H0ElF (ORCPT ); Mon, 27 Aug 2012 00:41:05 -0400 Received: from mga11.intel.com ([192.55.52.93]:13382 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751117Ab2H0ElD (ORCPT ); Mon, 27 Aug 2012 00:41:03 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.80,318,1344236400"; d="scan'208";a="213371320" Date: Mon, 27 Aug 2012 12:40:52 +0800 From: Fengguang Wu To: "Paul E. McKenney" Cc: Josh Triplett , Lai Jiangshan , linux-kernel@vger.kernel.org Subject: Re: INFO: suspicious RCU usage in rcu_torture_writer() Message-ID: <20120827044052.GA16267@localhost> References: <20120825033623.GA19330@localhost> <20120826000149.GG3436@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120826000149.GG3436@linux.vnet.ibm.com> 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 On Sat, Aug 25, 2012 at 05:01:49PM -0700, Paul E. McKenney wrote: > On Sat, Aug 25, 2012 at 11:36:23AM +0800, Fengguang Wu wrote: > > Greetings, > > > > I got this warning on 3.6.0-rc2. Full dmesg/config attached. > > > > [ 3.051375] Initializing RT-Tester: OK > > [ 3.052491] rcu-torture:--- Start of test: nreaders=2 nfakewriters=4 stat_interval=0 verbose=0 test_no_idle_hz=0 shuffle_interval=3 stut > > ter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/1 test_boost_interval=7 test_boost_duration=4 shutdown_secs=0 onoff_interval=0 onoff_holdoff=0 > > [ 3.059084] > > [ 3.059451] =============================== > > [ 3.060454] [ INFO: suspicious RCU usage. ] > > [ 3.061482] 3.6.0-rc2-00010-g4c58c42 #59 Not tainted > > [ 3.062686] ------------------------------- > > [ 3.063744] /c/kernel-tests/src/stable/kernel/rcutorture.c:990 suspicious rcu_dereference_check() usage! > > > > 982 do { > > 983 schedule_timeout_uninterruptible(1); > > 984 rp = rcu_torture_alloc(); > > 985 if (rp == NULL) > > 986 continue; > > 987 rp->rtort_pipe_count = 0; > > 988 udelay(rcu_random(&rand) & 0x3ff); > > 989 old_rp = rcu_dereference_check(rcu_torture_current, > > >990 current == writer_task); > > 991 rp->rtort_mbtest = 1; > > 992 rcu_assign_pointer(rcu_torture_current, rp); > > 993 smp_wmb(); /* Mods to old_rp must follow rcu_assign_pointer() */ > > 994 if (old_rp) { > > > Does the following clear this up? Sorry I'm still trying to reproduce this. It must be a rare bug because it only showed up in several of the tens of thousands of test boots. To reproduce it, I've done near 1000 boots however still not caught it yet. Let's run it for more time... Thanks, Fengguang > Thanx, Paul > > ------------------------------------------------------------------------ > > rcu: Prevent initialization race in rcutorture kthreads > > When you do something like "t = kthread_run(...)", it is possible that > the kthread will start running before the assignment to "t" happens. > If the child kthread expects to find a pointer to its task_struct in "t", > it will then be fatally disappointed. This commit therefore switches > such cases to kthread_create() followed by wake_up_process(), guaranteeing > that the assignment happens before the child kthread starts running. > > Reported-by: Fengguang Wu > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c > index 7a97b5b..8ff4fad 100644 > --- a/kernel/rcutorture.c > +++ b/kernel/rcutorture.c > @@ -2028,14 +2028,15 @@ rcu_torture_init(void) > /* Start up the kthreads. */ > > VERBOSE_PRINTK_STRING("Creating rcu_torture_writer task"); > - writer_task = kthread_run(rcu_torture_writer, NULL, > - "rcu_torture_writer"); > + writer_task = kthread_create(rcu_torture_writer, NULL, > + "rcu_torture_writer"); > if (IS_ERR(writer_task)) { > firsterr = PTR_ERR(writer_task); > VERBOSE_PRINTK_ERRSTRING("Failed to create writer"); > writer_task = NULL; > goto unwind; > } > + wake_up_process(writer_task); > fakewriter_tasks = kzalloc(nfakewriters * sizeof(fakewriter_tasks[0]), > GFP_KERNEL); > if (fakewriter_tasks == NULL) { > @@ -2150,14 +2151,15 @@ rcu_torture_init(void) > } > if (shutdown_secs > 0) { > shutdown_time = jiffies + shutdown_secs * HZ; > - shutdown_task = kthread_run(rcu_torture_shutdown, NULL, > - "rcu_torture_shutdown"); > + shutdown_task = kthread_create(rcu_torture_shutdown, NULL, > + "rcu_torture_shutdown"); > if (IS_ERR(shutdown_task)) { > firsterr = PTR_ERR(shutdown_task); > VERBOSE_PRINTK_ERRSTRING("Failed to create shutdown"); > shutdown_task = NULL; > goto unwind; > } > + wake_up_process(shutdown_task); > } > i = rcu_torture_onoff_init(); > if (i != 0) {