Consider the following case. Two threads are executing on a system. Thread 1 is reading from /sys/.../cpufreq/<files> and thread 2 is changing the cpufreq governor through /sys/.../cpufreq/scaling_governor.
Thread 2 acquires the mutexes in the write path, cpufreq_rwsem and policy->rwsem, while thread 1 waits. Thread 2 completes the changing of the scaling governor and releases the the mutexes. Thread 1 now acquires the mutexes and returns incorrect data as the governor has changed. The kernel cannot guarantee the governor from which the data came from, so the kernel should fail with -EBUSY when the governor is being written. Changing the down_read(&policy->rwsem) to a trylock fixes this stale data issue. Cc: "Rafael J. Wysocki" <[email protected]> Cc: Viresh Kumar <[email protected]> Cc: [email protected] Signed-off-by: Prarit Bhargava <[email protected]> --- drivers/cpufreq/cpufreq.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index 644b54e..3f09ca9 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -765,7 +765,8 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf) if (!down_read_trylock(&cpufreq_rwsem)) return -EINVAL; - down_read(&policy->rwsem); + if (!down_read_trylock(&policy->rwsem)) + return -EBUSY; if (fattr->show) ret = fattr->show(policy, buf); -- 1.7.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
