I wrote:
I'm attaching a patch which changes a semaphore in cpufreq into a
completion. With this patch, my system runs OK even with cpufreqd.
Although the patch worked for me, it was probably bogus.
The real reason why cpufreq caused problems was that it does:
init_MUTEX_LOCKED(&policy->lock);
and later:
up(&policy->lock);
where policy->lock is declared as:
struct semaphore lock;
In PREEMPT_RT, the init_MUTEX_LOCKED is defined in include/linux/rt_lock.h :
/*
* No locked initialization for RT semaphores:
*/
#define init_MUTEX_LOCKED(sem) compat_init_MUTEX_LOCKED(sem)
(BTW, I don't understand why we have init_MUTEX but no init_MUTEX_LOCKED
for RT semaphores).
Therefore we have an inconsistency - the up() gets translated into
rt_up() because the lock is of type struct semaphore. However,
compat_init_MUTEX_LOCKED assumes that it has a struct compat_semaphore.
The compiler actually warns about incompatible pointers but I missed
that warning at first.
So the fix is to change the lock type into compat_semaphore. I'm
attaching the patch. It works for me with 2.6.12-rc5-RT-V0.7.47-12.
Regards
Michal
diff -Nurp -X linux-RT4/Documentation/dontdiff linux-RT4/include/linux/cpufreq.h linux-RT3/include/linux/cpufreq.h
--- linux-RT4/include/linux/cpufreq.h 2005-05-27 22:00:11.000000000 +0200
+++ linux-RT3/include/linux/cpufreq.h 2005-05-27 21:54:52.000000000 +0200
@@ -81,7 +81,7 @@ struct cpufreq_policy {
unsigned int policy; /* see above */
struct cpufreq_governor *governor; /* see below */
- struct semaphore lock; /* CPU ->setpolicy or ->target may
+ struct compat_semaphore lock; /* CPU ->setpolicy or ->target may
only be called once a time */
struct work_struct update; /* if update_policy() needs to be
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