On 08/02/2013 12:51 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:31:23 AM Srivatsa S. Bhat wrote:
On 08/02/2013 12:31 AM, Rafael J. Wysocki wrote:
On Thursday, August 01, 2013 11:36:49 PM Srivatsa S. Bhat wrote:
Its the cpufreq_cpu_get() hidden away in cpufreq_add_dev_symlink(). With that taken care of, everything should be OK. Then we can change the synchronization part to avoid using refcounts.
So I actually don't see why cpufreq_add_dev_symlink() needs to call cpufreq_cpu_get() at all, since the policy refcount is already 1 at the point it is called and the bumping up of the driver module refcount is pointless.
Hmm, yes, it seems so.
However, if I change that I also need to change the piece of code that calls the complementary cpufreq_cpu_put() and I kind of cannot find it.
... I guess that's because you are looking at the code with your patch applied (and your patch removed that _put()) ;-)
No, it's not that one. That one was complementary to the cpufreq_cpu_get() done by cpufreq_add_policy_cpu() before my patch. Since my patch changes cpufreq_add_policy_cpu() to call cpufreq_cpu_put() before returning and bump up the policy refcount with kobject_get(), the one in __cpufreq_remove_dev() is changed into kobject_put() (correctly, IMO).
What gives?
Actually, it _is_ the one I pointed above. This thing is tricky, here's why:
cpufreq_add_policy_cpu() is called only if: a. The CPU being onlined has per_cpu(cpufreq_cpu_data, cpu) == NULL and b. Its is present in some CPU's related_cpus mask.
If condition (a) doesn't hold good, you get out right in the beginning of __cpufreq_add_dev().
So, cpufreq_add_policy_cpu() is called very rarely because, inside __cpufreq_add_dev we do:
1093 write_lock_irqsave(&cpufreq_driver_lock, flags); 1094 for_each_cpu(j, policy->cpus) { 1095 per_cpu(cpufreq_cpu_data, j) = policy; 1096 per_cpu(cpufreq_policy_cpu, j) = policy->cpu; 1097 } 1098 write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So for all the CPUs in the above policy->cpus mask, we simply return without further ado when they are onlined. In particular, we *dont* call cpufreq_add_policy_cpu() for any of them.
And their refcounts are incremented by the cpufreq_add_dev_interface()-> cpufreq_add_dev_symlink() function.
So, ultimately, we increment the refcount for a given non-policy-owner CPU only once. *Either* in cpufreq_add_dev_symlink() *or* in cpufreq_add_policy_cpu(), but never both.
So, in the teardown path, __cpufreq_remove_dev() needs only one place to decrement it as shown below:
1303 } else { 1304 1305 if (!frozen) { 1306 pr_debug("%s: removing link, cpu: %d\n", __func__, cpu); 1307 cpufreq_cpu_put(data); 1308 }
Pretty good maze, right? ;-(
Regards, Srivatsa S. Bhat