Re: [RFC][PATCH] cpufreq: Do not hold driver module references for additional policy CPUs
From: Rafael J. Wysocki
Date: Thu Aug 01 2013 - 15:54:01 EST
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.
Which entirely boils down to something like this:
---
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);
cpu_dev = get_cpu_device(j);
ret = sysfs_create_link(&cpu_dev->kobj, &policy->kobj,
"cpufreq");
- if (ret) {
- cpufreq_cpu_put(policy);
- return ret;
- }
+ if (ret)
+ break;
}
return ret;
}
@@ -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);
@@ -1298,12 +1287,6 @@ static int __cpufreq_remove_dev(struct d
if (!frozen)
cpufreq_policy_free(data);
} else {
-
- if (!frozen) {
- pr_debug("%s: removing link, cpu: %d\n", __func__, cpu);
- cpufreq_cpu_put(data);
- }
-
if (cpufreq_driver->target) {
__cpufreq_governor(data, CPUFREQ_GOV_START);
__cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
--
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