Re: [Update][PATCH] cpufreq: Do not hold driver module references for additional policy CPUs
From: Viresh Kumar <viresh.kumar@linaro.org>
Date: 2013-08-02 04:37:26
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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 :) On 2 August 2013 02:23, Rafael J. Wysocki [off-list ref] wrote:
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()?
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.
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. :(
quoted hunk ↗ jump to hunk
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()..
quoted hunk ↗ jump to hunk
- 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.