linux-kernel - Re: [PATCH] PM / OPP: Protect updates to list

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Message-ID: <20151105084214.GC3773@ubuntu>
Date:	Thu, 5 Nov 2015 14:12:14 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>, mturquette@...libre.com,
	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Dmitry Torokhov <dtor@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Len Brown <len.brown@...el.com>,
	open list <linux-kernel@...r.kernel.org>,
	Nishanth Menon <nm@...com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH] PM / OPP: Protect updates to list_dev with mutex

On 02-11-15, 11:14, Stephen Boyd wrote:
> On 10/31, Viresh Kumar wrote:
> > I don't know what is wrong (or right) with my exynos 5250 board, but I
> > didn't got any splat here even with the right config options (yes I
> > should have mentioned that earlier). I have seen this at other times
> > as well, while we were running after some cpufreq traces..
> > 
> > But, the case in hand is pretty straight forward and Mike T. did get a
> > splat as that's what he told me. We are calling a sleep-able function
> > from rcu_lock and that's obviously wrong.
> 
> That's slightly concerning. Given that the bug is so straight
> forward but we can't reproduce it doesn't instill a lot of
> confidence that the patch is correct.

Good that I spent some time debugging why I wasn't able to hit the
lockdep. And that's because by default CONFIG_PREEMPT_RCU was
selected on exynos and so the dummy function was getting enabled, and
so no WARN.

#if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
static inline void rcu_preempt_sleep_check(void)
{
	RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
			 "Illegal context switch in RCU read-side critical section");
}
#else /* #ifdef CONFIG_PROVE_RCU */
static inline void rcu_preempt_sleep_check(void)
{
}
#endif /* #else #ifdef CONFIG_PROVE_RCU */

Whereas for omap, that's not the case :)

So, with some config changes, I was able to hit the lockdep :)

And just to confirm, they got fixed by the $Subject patch :)

-- 
viresh
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