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Date: Thu, 1 Dec 2016 06:52:35 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Petr Mladek <pmladek@suse.com>,
        Vince Weaver <vincent.weaver@maine.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        "dvyukov@google.com" <dvyukov@google.com>
Subject: Re: perf: fuzzer BUG: KASAN: stack-out-of-bounds in __unwind_start
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On Wed, Nov 30, 2016 at 01:13:03PM -0600, Josh Poimboeuf wrote:
> This question was probably intended for other folks, but I should point
> out that idle tasks *do* invoke the scheduler.  cpu_idle_loop() calls
> schedule_preempt_disabled().

Right, but that doesn't matter I think. The below will simply not call
rcu_note_voluntary_context_switch() from the idle task, which would be
fine I think.

> > So is the following a sensible approach, or should I look elsewhere?
> > 
> > 	#define cond_resched_rcu_qs() \
> > 	do { \
> > 		if (!is_idle_task(current) && !cond_resched()) \
> > 			rcu_note_voluntary_context_switch(current); \

You should reverse your conditions though:

		if (!cond_resched() && !is_idle_task(current))
			rcu_note_voluntary_context_switch(current);

That way we'll still do cond_resched() and you only gate the RCU call.