On 08/02/2013 10:07 AM, Viresh Kumar wrote:

Wow!! Lot of stuff happened while I was asleep..

@Srivatsa: Thanks for answering what I would have answered to Rafael :)
And you should really get some sleep, I would suggest :)

No problem :-) And thank you for your concern :-)

On 2 August 2013 02:23, Rafael J. Wysocki [off-list ref] wrote:

quoted

From: Rafael J. Wysocki <redacted>
Subject: cpufreq: Do not hold driver module references for additional policy CPUs

I still have issues with this subject. Why don't we get rid of .owner
field completely? And stop using a mix of cpufreq_cpu_get() and
kobject_get()?

I guess Rafael's intention is to do one thing at a time - fix the
inconsistency first, and then rework the synchronization on top of
it. And that makes sense to me, since its the logical way of fixing
all these issues.
 

quoted

The cpufreq core is a little inconsistent in the way it uses the
driver module refcount.

Namely, if __cpufreq_add_dev() is called for a CPU that doesn't
share the policy object with any other CPUs, the driver module
refcount it grabs to start with will be dropped by it before
returning and will be equal to 0 afterward.

It wouldn't be zero but 1, this is what it is initialized with probably.
That's what I can see in my tests.

But lsmod shows 0 for the cpufreq driver right? (Note, your related_cpus
should have only 1 CPU each, for you to see 0. Else, you'll see a non-zero
value due to the very bug/inconsistency that Rafael is fixing in this
patch).

quoted

However, if the given CPU does share the policy object with other
CPUs, either cpufreq_add_policy_cpu() is called to link the new CPU
to the existing policy, or cpufreq_add_dev_symlink() is used to link
the other CPUs sharing the policy with it to the just created policy
object.  In that case, because both cpufreq_add_policy_cpu() and
cpufreq_add_dev_symlink() call cpufreq_cpu_get() for the given
policy (the latter possibly many times) without the balancing
cpufreq_cpu_put() (unless there is an error), the driver module
refcount will be left by __cpufreq_add_dev() with a nonzero value.

To remove that inconsistency make cpufreq_add_policy_cpu() execute
cpufreq_cpu_put() for the given policy before returning, which
decrements the driver module refcount so that it will be 0 after
__cpufreq_add_dev() returns.  Moreover, remove the cpufreq_cpu_get()
call from cpufreq_add_dev_symlink(), since both the policy refcount
and the driver module refcount are nonzero when it is called and they
don't need to be bumped up by it.

Accordingly, drop the cpufreq_cpu_put() from __cpufreq_remove_dev(),
since it is only necessary to balance the cpufreq_cpu_get() called
by cpufreq_add_policy_cpu() or cpufreq_add_dev_symlink().

Signed-off-by: Rafael J. Wysocki <redacted>
Reviewed-by: Srivatsa S. Bhat <redacted>
---
 drivers/cpufreq/cpufreq.c |   28 +++++++---------------------
 1 file changed, 7 insertions(+), 21 deletions(-)

So, we can't rmmod the module as soon as it is inserted and so the
problem stays as is. :(

No, we get one step closer to the solution, since we fix the inconsistency
between refcounts. Next step would be to get rid of refcounts and use
locking like you suggested. Then we can rmmod it easily. I'm assuming
Rafael has the same plan.

(We could have done all this in one-shot, but that would make it difficult
to track regressions etc. So good to have each improvement in a separate
patch).

quoted

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);

This can be skipped completely at this place. Caller of
cpufreq_add_policy_cpu() has got the policy pointer with it and so
can be passed. I haven't done it earlier as the impression was we need
to call cpufreq_cpu_get()..

Agreed, that would be a good cleanup.

quoted

-       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);

And so this will go away.

Regards,
Srivatsa S. Bhat