Date: Wed, 30 Nov 2016 11:49:57 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, 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 Wed, Nov 30, 2016 at 01:13:03PM -0600, Josh Poimboeuf wrote:
> On Tue, Nov 29, 2016 at 12:32:59PM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 29, 2016 at 12:07:11PM -0800, Paul E. McKenney wrote:
> > > On Tue, Nov 29, 2016 at 08:52:04PM +0100, Peter Zijlstra wrote:
> > > > On Tue, Nov 29, 2016 at 11:39:35AM -0800, Paul E. McKenney wrote:
> > > > > On Tue, Nov 29, 2016 at 06:10:38PM +0100, Peter Zijlstra wrote:
> > > > 
> > > > > > It mostly works, most of the time, and that seems to be what Linus
> > > > > > wants, since its really the best we can have given the constraints. But
> > > > > > for debugging, when you have a UART, it totally blows.
> > > > > 
> > > > > UART???  They still make those things???  ;-)
> > > > 
> > > > Yes, most computer like devices actually have them, trouble is, most
> > > > consumer devices don't have the pins exposed. Luckily most server class
> > > > hardware still does.
> > > > 
> > > > And they're absolutely _awesome_ for debugging; getting data out is a
> > > > matter of trivial MMIO poll loops. Rock solid stuff.
> > > 
> > > They very clearly need to bring the baud rate into the current millenium,
> > > many tens of Mbaud at the -very- least.
> > 
> > On a more practical note...
> > 
> > Currently, cond_resched_rcu_qs() is not permitted to be invoked until
> > after the scheduler has started.  However, it appears that there is some
> > kernel code that can loop for quite some time at runtime, but which also
> > executes during early boot.  So it would be good to make it so that
> > cond_resched_rcu_qs() could be called at boot.
> > 
> > One approach would be to check rcu_scheduler_active, but this isn't
> > defined in normal Tiny RCU builds.  I can expand Tiny RCU, or I can
> > kludge the non-CONFIG_DEBUG_LOCK_ALLOC value of rcu_scheduler_active
> > to false (with this latter being the current state).  But it occurred
> > to me that I could also condition on !is_idle_task(), given that idle
> > tasks shouldn't ever be invoking the scheduler anyway.
> 
> 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().

Good point.  My next fallback is that idle loops should not be running
for long periods of time within RCU_NONIDLE().  Does that work?

							Thanx, Paul

> > 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); \
> > 	} while (0)
> > 
> > 							Thanx, Paul
> > 
> 
> -- 
> Josh
>