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[PATCH RT] Align rt_mutex inlining with upstream behavior

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  [PATCH RT] Align rt_mutex inlining with upstream behavior Alex Goins <agoins@nvidia.com> - 2017-01-25 03:50 +0100
    Re: [PATCH RT] Align rt_mutex inlining with upstream behavior Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-01-26 18:10 +0100
      Re: [PATCH RT] Align rt_mutex inlining with upstream behavior Andy Ritger <aritger@nvidia.com> - 2017-01-30 18:40 +0100

#1566289 — [PATCH RT] Align rt_mutex inlining with upstream behavior

mutex_destroy is no-op inline when DEBUG_MUTEX is not enabled. The RT Linux
patches replace mutex_destroy() with rt_mutex_destroy(). This patch aligns
rt_mutex_destroy() with mutex_destroy() by using the same no-op inline
technique.

Signed-off-by: Alex Goins <agoins@nvidia.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>
---
 include/linux/rtmutex.h  | 7 ++++++-
 kernel/locking/rtmutex.c | 5 ++---
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/rtmutex.h b/include/linux/rtmutex.h
index 1abba5c..741e844 100644
--- a/include/linux/rtmutex.h
+++ b/include/linux/rtmutex.h
@@ -56,6 +56,12 @@ struct rt_mutex {
 #endif
 
 #ifdef CONFIG_DEBUG_RT_MUTEXES
+ extern void rt_mutex_destroy(struct rt_mutex *lock);
+#else
+ static inline void rt_mutex_destroy(struct rt_mutex *lock) {}
+#endif
+
+#ifdef CONFIG_DEBUG_RT_MUTEXES
 # define __DEBUG_RT_MUTEX_INITIALIZER(mutexname) \
 	, .name = #mutexname, .file = __FILE__, .line = __LINE__
 # define rt_mutex_init(mutex)			__rt_mutex_init(mutex, __func__)
@@ -87,7 +93,6 @@ static inline int rt_mutex_is_locked(struct rt_mutex *lock)
 }
 
 extern void __rt_mutex_init(struct rt_mutex *lock, const char *name);
-extern void rt_mutex_destroy(struct rt_mutex *lock);
 
 extern void rt_mutex_lock(struct rt_mutex *lock);
 extern int rt_mutex_lock_interruptible(struct rt_mutex *lock);
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2c49d76..8ff12fb 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1577,6 +1577,7 @@ bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
 	return rt_mutex_slowunlock(lock, wqh);
 }
 
+#ifdef CONFIG_DEBUG_RT_MUTEXES
 /**
  * rt_mutex_destroy - mark a mutex unusable
  * @lock: the mutex to be destroyed
@@ -1588,12 +1589,10 @@ bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
 void rt_mutex_destroy(struct rt_mutex *lock)
 {
 	WARN_ON(rt_mutex_is_locked(lock));
-#ifdef CONFIG_DEBUG_RT_MUTEXES
 	lock->magic = NULL;
-#endif
 }
-
 EXPORT_SYMBOL_GPL(rt_mutex_destroy);
+#endif
 
 /**
  * __rt_mutex_init - initialize the rt lock
-- 
1.9.1

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#1567609

On 2017-01-24 18:45:50 [-0800], Alex Goins wrote:
> mutex_destroy is no-op inline when DEBUG_MUTEX is not enabled. The RT Linux
> patches replace mutex_destroy() with rt_mutex_destroy(). This patch aligns
> rt_mutex_destroy() with mutex_destroy() by using the same no-op inline
> technique.
> 
> Signed-off-by: Alex Goins <agoins@nvidia.com>
> Reviewed-by: Andy Ritger <aritger@nvidia.com>

So what is the problem? Why are we doing this? There is still a check to
see if the lock is in use which is also done for the case where
DEBUG_MUTEX is disabled.

Sebastian

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#1569943

On Thu, Jan 26, 2017 at 06:01:09PM +0100, Sebastian Andrzej Siewior wrote:
> On 2017-01-24 18:45:50 [-0800], Alex Goins wrote:
> > mutex_destroy is no-op inline when DEBUG_MUTEX is not enabled. The RT Linux
> > patches replace mutex_destroy() with rt_mutex_destroy(). This patch aligns
> > rt_mutex_destroy() with mutex_destroy() by using the same no-op inline
> > technique.
> > 
> > Signed-off-by: Alex Goins <agoins@nvidia.com>
> > Reviewed-by: Andy Ritger <aritger@nvidia.com>
> 
> So what is the problem? Why are we doing this? There is still a check to
> see if the lock is in use which is also done for the case where
> DEBUG_MUTEX is disabled.

The problem is that various static inline functions such as
reservation_object_fini() indirectly call mutex_destroy.  On DEBUG_MUTEX
kernels, mutex_destroy is EXPORT_SYMBOL_GPL.

In upstream, non-DEBUG_MUTEX kernels define mutex_destroy to a noop.
This gives users the option of disabling DEBUG_MUTEX if they want to
use non-GPL, reservation_object_fini()-using, kernel modules.

In PREEMPTRT, non-DEBUG_MUTEX kernels export rt_mutex_destroy as
EXPORT_SYMBOL_GPL, so users no longer have the work around of using
DEBUG_MUTEX.

This patch gives PREEMPTRT users the option of disabling DEBUG_MUTEX if
they want to use such kernel modules, matching upstream behavior.

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