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SubjectRe: [PATCH] [CPUFREQ] fix race condition in store_scaling_governor
On Tue, May 11, 2010 at 04:20:41PM +0200, Andrej Gelenberg wrote:
>Wrap store_scaling_governor with mutex lock cpufreq_governor_mutex.
>Fix kernel panic if switch scaling governor very fast.
>Bug in bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=15948
>
>Signed-off-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
>---
> drivers/cpufreq/cpufreq.c | 16 +++++++++++-----
> 1 files changed, 11 insertions(+), 5 deletions(-)
>
>diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>index 75d293e..6ba42f9 100644
>--- a/drivers/cpufreq/cpufreq.c
>+++ b/drivers/cpufreq/cpufreq.c
>@@ -403,8 +403,6 @@ static int cpufreq_parse_governor(char
>*str_governor, unsigned int *policy,
> } else if (cpufreq_driver->target) {
> struct cpufreq_governor *t;
>
>- mutex_lock(&cpufreq_governor_mutex);
>-
> t = __find_governor(str_governor);
>
> if (t == NULL) {
>@@ -429,8 +427,6 @@ static int cpufreq_parse_governor(char
>*str_governor, unsigned int *policy,
> *governor = t;
> err = 0;
> }
>-
>- mutex_unlock(&cpufreq_governor_mutex);
> }
> out:
> return err;
>@@ -521,7 +517,7 @@ static ssize_t show_scaling_governor(struct
>cpufreq_policy *policy, char *buf)
> /**
> * store_scaling_governor - store policy for the specified CPU
> */
>-static ssize_t store_scaling_governor(struct cpufreq_policy *policy,
>+static ssize_t _store_scaling_governor(struct cpufreq_policy *policy,
> const char *buf, size_t count)
> {
> unsigned int ret = -EINVAL;
>@@ -553,6 +549,16 @@ static ssize_t store_scaling_governor(struct
>cpufreq_policy *policy,
> return count;
> }
>
>+static ssize_t store_scaling_governor(struct cpufreq_policy *policy,
>+ const char *buf, size_t count)
>+{
>+ ssize_t ret;
>+ mutex_lock(&cpufreq_governor_mutex);
>+ ret = _store_scaling_governor(policy, buf, count);
>+ mutex_unlock(&cpufreq_governor_mutex);
>+ return ret;
>+}
>+

Sorry, I don't get it, cpufreq_governor_mutex is used to protect
cpufreq_governor_list. What is the point of moving it up?
Can you explain what the race condition is?

Thanks!



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