Re: [patch] Real-Time Preemption, -RT-2.6.12-rc4-V0.7.47-06

[Michal Schmidt]

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