From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751847Ab1GONIE (ORCPT ); Fri, 15 Jul 2011 09:08:04 -0400 Received: from casper.infradead.org ([85.118.1.10]:35564 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750865Ab1GONIC convert rfc822-to-8bit (ORCPT ); Fri, 15 Jul 2011 09:08:02 -0400 Subject: Re: INFO: possible circular locking dependency detected From: Peter Zijlstra To: paulmck@linux.vnet.ibm.com Cc: Ed Tomlinson , Steven Rostedt , Sergey Senozhatsky , Ingo Molnar , Thomas Gleixner , Andrew Morton , Dipankar Sarma , linux-kernel@vger.kernel.org In-Reply-To: <20110715124206.GA2376@linux.vnet.ibm.com> References: <20110714144946.GA3354@swordfish.minsk.epam.com> <1310665613.27864.50.camel@gandalf.stny.rr.com> <20110714191809.GF2349@linux.vnet.ibm.com> <201107150705.46248.edt@aei.ca> <1310729362.2586.325.camel@twins> <20110715124206.GA2376@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 15 Jul 2011 15:07:39 +0200 Message-ID: <1310735259.2586.330.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2011-07-15 at 05:42 -0700, Paul E. McKenney wrote: > On Fri, Jul 15, 2011 at 01:29:22PM +0200, Peter Zijlstra wrote: > > On Fri, 2011-07-15 at 07:05 -0400, Ed Tomlinson wrote: > > > Jul 14 23:21:18 grover kernel: [ 920.659426] -> #1 (rcu_node_level_0){..-...}: > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] lock_acquire+0x95/0x140 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] _raw_spin_lock+0x3b/0x50 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] __rcu_read_unlock+0x197/0x2d0 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] select_task_rq_fair+0x585/0xa80 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] try_to_wake_up+0x17b/0x360 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] wake_up_process+0x15/0x20 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] irq_exit+0xb4/0x100 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] smp_apic_timer_interrupt+0x6e/0x99 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] apic_timer_interrupt+0x13/0x20 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] __rcu_read_unlock+0xe9/0x2d0 > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [] sock_def_readable+0x94/0xc0 > > > > Ed, are you perchance running with force_irqthreads? > > > > Paul, what appears to be happening here is that some rcu_read_unlock() > > gets interrupted, possibly before calling rcu_read_unlock_special(), > > possibly not if the interrupt is itself the timer interrupt. > > > > Supposing ->rcu_read_unlock_special is set before, any wakeup happening > > from an interrupt hitting __rcu_read_unlock(): > > > > void __rcu_read_unlock(void) > > { > > struct task_struct *t = current; > > > > barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */ > > --t->rcu_read_lock_nesting; > > barrier(); /* decrement before load of ->rcu_read_unlock_special */ > > if (t->rcu_read_lock_nesting == 0 && > > unlikely(ACCESS_ONCE(t->rcu_read_unlock_special))) > > rcu_read_unlock_special(t); > > #ifdef CONFIG_PROVE_LOCKING > > WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); > > #endif /* #ifdef CONFIG_PROVE_LOCKING */ > > } > > > > After --t->rcu_read_lock_nesting, but before calling > > rcu_read_unlock_special(), will trigger this lock inversion. > > > > The alternative case, ->rcu_read_unlock_special is not set yet, it can > > be set if the interrupt hitting in that same spot above, is the timer > > interrupt, and the wakeup happens either from the softirq ran from the > > hard IRQ tail, or as I suspect here happens, the wakeup of ksoftirqd/#. OK, so the latter case cannot happen (rcu_preempt_check_callbacks only sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for this to happen. rcu_read_lock() |= RCU_READ_UNLOCK_NEED_QS rcu_read_unlock() __rcu_read_unlock() --rcu_read_lock_nesting; ttwu() rcu_read_lock() rcu_read_unlock() rcu_read_unlock_special() *BANG* rcu_read_unlock_special()