Date: Thu, 1 Dec 2016 04:33:16 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.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
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Dec 01, 2016 at 06:52:35AM +0100, Peter Zijlstra wrote:
> 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.

This makes it illegal at early boot.  This is not a problem with the
surviving cond_resched_rcu_qs(), but one of the candidates really was
called at boot time.  If I reverse the order as you say, I can just as
well leave of the "!is_idle_task(current)".

So I will just drop this patch until such time as someone actually needs
to add a cond_resched_rcu_qs() that sometimes gets invoked at boot time.

							Thanx, Paul