Messages in this thread | | | Date | Thu, 25 Jun 2009 17:32:19 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 2/3] cpufreq: Define dbs_mutex purpose and cleanup its usage |
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* Pallipadi, Venkatesh (venkatesh.pallipadi@intel.com) wrote: > On Thu, 2009-06-25 at 12:46 -0700, Mathieu Desnoyers wrote: > > * venkatesh.pallipadi@intel.com (venkatesh.pallipadi@intel.com) wrote: > > > Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very > > > much needed to cleanup ondemand timer cleanly, openup a can of worms > > > related to locking dependencies in cpufreq. > > > > > > Patch here defines the need for dbs_mutex and cleans up its usage in > > > ondemand governor. This also resolves the lockdep warnings reported here > > > > > > http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html > > > > > > > @@ -598,14 +593,16 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy, > > > max(min_sampling_rate, > > > latency * LATENCY_MULTIPLIER); > > > } > > > + mutex_unlock(&dbs_mutex); > > > + > > > dbs_timer_init(this_dbs_info); > > > > > > - mutex_unlock(&dbs_mutex); > > > break; > > > > > > case CPUFREQ_GOV_STOP: > > > - mutex_lock(&dbs_mutex); > > > dbs_timer_exit(this_dbs_info); > > > > Hrm, so.. how do we protect against concurrent : > > > > CPUFREQ_GOV_START/CPUFREQ_GOV_STOP now ? > > concurrent _START _STOP across CPUs does not matter for timer_init and > timer_exit.
Given those are per-cpu anyway I guess. Hopefully it works OK with CPU hotplug.
> On same CPU, there cannot be two concurrent _START as upper > level cpufreq will have policy_rwsem in write mode.
Agreed.
> I cannot think of a > flow where _START and _STOP on same CPU is possible. >
_STOP is not protected by any mutex now. So it could be preempted, and then a _START executed, and there is your race.
> However two concurrent _STOP for same CPU is still possible, as we are > releasing the rwsem lock before STOP callback. "Back to drawing board" > time to figure this all out..
I fear that it is indeed the case. If you can come up with a document explaining the expected interactions between :
- cpu hotplug - policy lock - cpufreq driver lock - timer lock
that would be awesome. :)
Mathieu
> > Thanks, > Venki >
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