Messages in this thread | | | Subject | Re: INFO: possible circular locking dependency detected | From | Peter Zijlstra <> | Date | Fri, 15 Jul 2011 18:11:55 +0200 |
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On Fri, 2011-07-15 at 08:59 -0700, Paul E. McKenney wrote:
> > Because we're in irq_exit(), after decrementing preempt_count, so > > in_irq() returns false. > > Can we delay decrementing preempt_count so that RCU has some chance > of actually working?
No, softirqs must be ran with in_irq() being false.
> > No, the *BANG* being that we end up calling rcu_read_unlock_special() > > while holding scheduler locks, which is BAD(tm). > > Well, it certainly is BAD(tm) if you guys continue to deprive > rcu_read_unlock_special() of the means of determining whether it is > being invoked from hardware irq handler context.
hard irq handler isn't really the problem here, its the nested softirq code that is.
> > > (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(). > > Right. So how about delaying clearing those bits until after you get > done messing with the scheduler from hardware irq handler context?
Can't do.
> > 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(). > > Change the rules without adjusting the callers can in fact result in some > breakage. ;-)
There's no changing the rules here, this is how its worked for a very long time indeed. Softirqs can run from the hardirq tail.
> The bit about local_irq_save() and local_irq_restore() invoking the > scheduler is rather surprising -- is there a raw_ version that avoids > this?
They don't, they might for -rt, but that's a different story. But looking at the latest version I have its only local_irq_save_rt() and friends that do that.
> > > 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 */ > > > > <preempt> |= 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() > > Why can't we exchange the order of the above two so that RCU correctly > avoids messing with the scheduler if called from hardware interrupt > context?
Because softirqs != hardirq ? This has been so like forever, can't go change the semantics of this without risking tons of borkage. Every time we try to change softirq semantics (we tried with -rt, because softirqs are a massive pain) everything goes tits up fast.
> > > > 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. > > An easily reproduced bug is certainly a nice change of pace... > > > Something like the below _might_ fix it.. > > Maybe, but how does tglx make PREEMPT_RT work in this case? The problem > is that PREEMPT_RT allows ksoftirqd to be preempted, and thus allows it > to be RCU priority boosted.
RT is mostly easier since it doesn't nest as many contexts, softirqs for example always run in task context, and the only way to run them in a random tasks' context is through local_bh_enable() and since there's no local_bh_enable() call in the middle of __rcu_read_unlock() you're pretty good there.
I know tglx has some softirq changes he hasn't yet shared with me, but if the patch I send earlier fixes the problem for mainline, I'm fairly confident I can cook one up for him as well.
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