On Friday, August 02, 2013 01:56:21 AM Srivatsa S. Bhat wrote:
On 08/02/2013 01:34 AM, Rafael J. Wysocki wrote:
On Friday, August 02, 2013 12:51:24 AM Srivatsa S. Bhat wrote:
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? ;-(
Oh dear. Right.
I tgought I could change cpufreq_add_dev_symlink() to use kobject_get() to bump up the policy refcount in analogy with cpufreq_add_policy_cpu() and then it wouldn't need to call cpufreq_cpu_get() at all, but there is a bug in the error code path of cpufreq_add_dev_interface(), because if cpufreq_add_dev_symlink() fails for one of the CPUs sharing the policy, it will just fail to drop references grabbed in there. [Moreover, if it fails for the first one different from policy->cpu, kobject_put() will be called for that policy twice in a row if I'm not mistaken (first by cpufreq_add_dev_interface() and then by __cpufreq_add_dev()), but that's a different matter.]
So I think that neither cpufreq_add_dev_symlink() nor cpufreq_add_policy_cpu() should bump up the policy refcount in any way.
Yeah, that greatly simplifies things, as seen in the patch below.
Which entirely boils down to something like this:
Looks good to me.
Reviewed-by: Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com
Thanks! :-)
I actually think that I should move the error code path bug fix ->
drivers/cpufreq/cpufreq.c | 31 +++++++------------------------ 1 file changed, 7 insertions(+), 24 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c
--- linux-pm.orig/drivers/cpufreq/cpufreq.c +++ linux-pm/drivers/cpufreq/cpufreq.c @@ -818,14 +818,11 @@ static int cpufreq_add_dev_symlink(struc continue;
pr_debug("Adding link for CPU: %u\n", j);
cpufreq_cpu_get(policy->cpu);if (ret) {cpufreq_cpu_put(policy);return ret;}
if (ret)break;} @@ -908,7 +905,8 @@ static int cpufreq_add_policy_cpu(unsign unsigned long flags;
policy = cpufreq_cpu_get(sibling);
- WARN_ON(!policy);
if (WARN_ON_ONCE(!policy))
return -ENODATA;if (has_target) __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
@@ -930,16 +928,10 @@ static int cpufreq_add_policy_cpu(unsign }
/* Don't touch sysfs links during light-weight init */
- if (frozen) {
/* Drop the extra refcount that we took above */cpufreq_cpu_put(policy);return 0;- }
- ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");
- if (ret)
cpufreq_cpu_put(policy);
if (!frozen)
ret = sysfs_create_link(&dev->kobj, &policy->kobj, "cpufreq");cpufreq_cpu_put(policy); return ret;
} #endif @@ -1117,9 +1109,6 @@ err_out_unregister: } write_unlock_irqrestore(&cpufreq_driver_lock, flags);
- kobject_put(&policy->kobj);
- wait_for_completion(&policy->kobj_unregister);
err_set_policy_cpu: per_cpu(cpufreq_policy_cpu, cpu) = -1; cpufreq_policy_free(policy);
-> into a separate patch, because it's not really related to the other changes made here.
Thanks, Rafael