Messages in this thread | | | Date | Thu, 17 Nov 2016 19:27:09 +0530 | From | Viresh Kumar <> | Subject | Re: [PATCH v2] cpufreq: Avoid a couple of races related to cpufreq_cpu_get() |
| |
On 17-11-16, 14:35, Rafael J. Wysocki wrote: > That unless cpu == policy->cpu and it is going offline I suppose? > > The scenario is as follows. cpufreq_get() is invoked for policy->cpu > and cpufreq_offline() runs for it at the same time. > > cpufreq_get() calls cpufreq_cpu_get() which does the policy->cpus > check which passes, because cpufreq_offline() hasn't updated the mask > yet. Now cpufreq_offline() updates the mask and proceeds with > cpufreq_driver->stop_cpu() and cpufreq_driver->exit(). Then, it drops > the lock. > > cpufreq_get() acquires the lock. The policy is still there, but it > may be inactive at this point. Still, cpufreq_get() doesn't check > that, but invokes __cpufreq_get() unconditionally, which calls > cpufreq_driver->get(policy->cpu). Is this still guaranteed to work? > I don't think so. > > It looks like a policy_is_inactive() check should be there in > cpufreq_get() at least.
Okay, trying to do any operations on the device for an inactive policy is absolutely wrong. I agree.
> >> + > >> up_read(&policy->rwsem); > >> > >> cpufreq_cpu_put(policy); > >> @@ -2142,6 +2154,11 @@ int cpufreq_get_policy(struct cpufreq_po > >> if (!cpu_policy) > >> return -EINVAL; > >> > >> + if (!cpumask_test_cpu(cpu, policy->cpus)) { > >> + cpufreq_cpu_put(cpu_policy); > >> + return -EINVAL; > >> + } > >> + > > > > We are just copying the policy here, so it should be always safe. > > So the check is not necessary at all?
Right.
> Say the CPU is the only one in the policy and it is going offline. > > cpufreq_update_policy() is invoked at the same time and calls > cpufreq_cpu_get() which checks policy->cpus and the test passes, > because cpufreq_offline() hasn't updated the mask yet. The > cpufreq_offline() updates the mask and the policy becomes inactive, > but there are no checks for that going forward, unless Im overlooking > something again.
Same here. I agree.
> > Also, even if we have some real cases for cpufreq_cpu_get_raw(), which > > needs to get fixed, I believe that we can move the check to > > cpufreq_cpu_get() and not to every caller. > > I disagree, but for now I'm going to leave cpufreq_cpu_get() alone. > To me, the policy->cpus check in cpufreq_cpu_get_raw() is just > confusing (it isn't even needed by some callers of that function), > which is the reason why I'd prefer to get rid of it.
Okay.
> I'll add policy_is_inactive() checks to cpufreq_get() and > cpufreq_update_policy() at this point.
That would be much better I think.
-- viresh
| |