Messages in this thread | | | Date | Mon, 2 Nov 2015 11:14:35 -0800 | From | Stephen Boyd <> | Subject | Re: [PATCH] PM / OPP: Protect updates to list_dev with mutex |
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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.
> > > > 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.
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