From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752852Ab1GOPE5 (ORCPT ); Fri, 15 Jul 2011 11:04:57 -0400 Received: from casper.infradead.org ([85.118.1.10]:53175 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752019Ab1GOPEz convert rfc822-to-8bit (ORCPT ); Fri, 15 Jul 2011 11:04:55 -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: <20110715143651.GB2376@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> <1310735259.2586.330.camel@twins> <20110715143651.GB2376@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 15 Jul 2011 17:04:27 +0200 Message-ID: <1310742267.2586.353.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 07:36 -0700, Paul E. McKenney wrote: > On Fri, Jul 15, 2011 at 03:07:39PM +0200, Peter Zijlstra wrote: > > 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 > > Yow... Looks like rcu_read_unlock_special() is being inlined > into __rcu_read_unlock(). Yeah, *cheer* for gcc being a smarty-pants. > > > > > 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 > > OK, so all the above stuff is in the context of an irq handler, right? yep > In which case, why didn't the in_irq() check kick us out before we > had a chance to attempt to acquire any locks? Because we're in irq_exit(), after decrementing preempt_count, so in_irq() returns false. > > > > > 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? > > Ah! Would that mean that local_irq_save() gets mapped to locking? > Now -that- could be exciting! ;-) Nope, it simply makes the invoke_softirq() call in irq_exit() do an unconditional wakeup of ksoftirqd/# since there isn't an irq-tail to speak of. > > > > 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() > > The "*BANG*" indicating that the upper-level rcu_read_unlock_special() > might overwrite the lower-level rcu_read_unlock_special()'s attempt > to clear RCU_READ_UNLOCK_NEED_QS? No, the *BANG* being that we end up calling rcu_read_unlock_special() while holding scheduler locks, which is BAD(tm). > (Which I believe, perhaps > incorrectly, to be prevented by the fact that all modifications to > ->rcu_read_unlock_special are carried out with irqs disabled on the > corresponding CPU, at least given no RCU_BOOST.) The check for in_irq() > should prevent the from-irq rcu_read_unlock_special() from attempting > to acquire any locks. Right, so in_irq() simply checks a few bits in preempt_count, which we just cleared due to being in irq_exit(). > Or am I missing the point of your example? You were ;-) > On the other points, to the extent that I have analyzed them so far: > > 1. If the task is preempted after the --rcu_read_lock_nesting, > it won't see it as being in an RCU read-side critical section, > so won't queue it. Agreed. > 2. Of course, the task might have preempted earlier. In this > case, the RCU_READ_UNLOCK_BLOCKED is already set, so we will be > invoking rcu_read_unlock_special() anyway. Right, but > If an interrupt happens between the decrement and the call to > rcu_read_unlock_special(), then, yes, the irq handler will also > call rcu_read_unlock_special() if it calls rcu_read_unlock(), but > the check for in_irq() will prevent the irq handler's invocation > of rcu_read_unlock_special() from acquiring any locks. But in_irq() isn't sufficient for RCU usage after the hardirq ends, see irq_exit(). Also there's all of softirq to consider, that too can run and not get caught by in_irq(). > 3. It is possible that the task is preempted after the > --rcu_read_lock_nesting, in which case the task won't be queued. > Of course the task might already be queued if there was an > earlier preemption during this same RCU read-side critical > section, in which case #2 applies. > > In other words, a preemption in __rcu_read_unlock() after the > --rcu_read_lock_nesting has no effect on RCU state: either the > task was already marked RCU_READ_UNLOCK_BLOCKED, or it wasn't. > Either way, rcu_note_context_switch() does not see this task as > being in an RCU read-side critical section. > > So what am I missing here? $task IRQ SoftIRQ rcu_read_lock() /* do stuff */ |= UNLOCK_BLOCKED rcu_read_unlock() --t->rcu_read_lock_nesting irq_enter(); /* do stuff, don't use RCU */ irq_exit(); sub_preempt_count(IRQ_EXIT_OFFSET); invoke_softirq() ttwu(); spin_lock_irq(&pi->lock) rcu_read_lock(); /* do stuff */ rcu_read_unlock(); rcu_read_unlock_special() rcu_report_exp_rnp() ttwu() spin_lock_irq(&pi->lock) /* deadlock */ rcu_read_unlock_special(t); Ed can simply trigger this 'easy' because invoke_softirq() immediately does a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff first, but even without that the above happens. Something like the below _might_ fix it.. --- kernel/rcutree_plugin.h | 2 +- kernel/softirq.c | 12 ++++++++++-- 2 files changed, 11 insertions(+), 3 deletions(-) diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index 14dc7dd..373c9c8 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -309,7 +309,7 @@ static void rcu_read_unlock_special(struct task_struct *t) } /* Hardware IRQ handlers cannot block. */ - if (in_irq()) { + if (in_irq() || in_serving_softirq()) { local_irq_restore(flags); return; } diff --git a/kernel/softirq.c b/kernel/softirq.c index 40cf63d..fca82c3 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -315,16 +315,24 @@ static inline void invoke_softirq(void) { if (!force_irqthreads) __do_softirq(); - else + else { + __local_bh_disable((unsigned long)__builtin_return_address(0), + SOFTIRQ_OFFSET); wakeup_softirqd(); + __local_bh_enable(SOFTIRQ_OFFSET); + } } #else static inline void invoke_softirq(void) { if (!force_irqthreads) do_softirq(); - else + else { + __local_bh_disable((unsigned long)__builtin_return_address(0), + SOFTIRQ_OFFSET); wakeup_softirqd(); + __local_bh_enable(SOFTIRQ_OFFSET); + } } #endif