* Pallipadi, Venkatesh (venkatesh.pallipadi@xxxxxxxxx) wrote: > On Thu, 2009-06-25 at 12:46 -0700, Mathieu Desnoyers wrote: > > * venkatesh.pallipadi@xxxxxxxxx (venkatesh.pallipadi@xxxxxxxxx) 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 > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
