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

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Message-ID: <20151104021916.GA11738@ubuntu>
Date:	Wed, 4 Nov 2015 07:49:16 +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:
> > On 30-10-15, 10:06, Stephen Boyd wrote:
> > > On 10/30, Viresh Kumar wrote:
> > > > dev_opp_list_lock is used everywhere to protect device and OPP lists,
> > > > but dev_pm_opp_set_sharing_cpus() is missed somehow. And instead we used
> > > > rcu-lock, which wouldn't help here as we are adding a new list_dev.
> > > > 
> > > > This also fixes a problem where we have called kzalloc(..., GFP_KERNEL)
> > > > from within rcu-lock, which isn't allowed as kzalloc can sleep when
> > > > called with GFP_KERNEL.
> > > 
> > > Care to share the splat here?
> > 
> > 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.

I have asked Mike to provide the splat he got and test the new patch
to see if it is fixed or not.

> > > > diff --git a/drivers/base/power/opp/cpu.c b/drivers/base/power/opp/cpu.c
> > > > index 7654c5606307..91f15b2e25ee 100644
> > > > --- a/drivers/base/power/opp/cpu.c
> > > > +++ b/drivers/base/power/opp/cpu.c
> > > > @@ -124,12 +124,12 @@ int dev_pm_opp_set_sharing_cpus(struct device *cpu_dev, cpumask_var_t cpumask)
> > > >  	struct device *dev;
> > > >  	int cpu, ret = 0;
> > > >  
> > > > -	rcu_read_lock();
> > > > +	mutex_lock(&dev_opp_list_lock);
> > > >  
> > > >  	dev_opp = _find_device_opp(cpu_dev);
> > > 
> > > So does _find_device_opp() need to be called with rcu_read_lock()
> > > held or not? The comment above the function makes it sound like
> > > we need RCU, but we don't do that here anymore.
> > 
> > That is more for the readers, as this function is going to return a
> > pointer to the device OPP, and to make sure it isn't freed behind
> > their back, they need to take the RCU lock.
> > 
> > There are other writer code paths as well, like add-opp, where we just
> > take the mutex as there can't be anything stronger than that :)
> > 
> 
> Agreed, but the comment above the function is misleading. We
> should correct that comment and/or add the lockdep checks to the
> function like we have elsewhere in this file.

Will do.

-- 
viresh
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