Re: rt_mutex: restore wait_lock init in __rt_mutex_init

[Thomas Gleixner]

On Wed, 27 Jul 2011, Darren Hart wrote:

> Without the raw_spin_lock_init(), the wait_lock does not get properly
> initialized with CONFIG_DEBUG_SPINLOCK. This can manifest in a BUG() in the
> futex requeue_pi path when the pi_state->pi_mutex->wait_lock fails the magic
> test in rt_mutex_start_proxy_lock()->raw_spin_lock(&lock->wait_lock).

That's actively wrong. You reinitialize the lock for all other cases
which call this via rt_mutex_init(). There is a reason why I moved the
spin lock initializer out of __rt_mutex_init() into
rt_mutex_init(). The lock name stuff for lockdep ends up to be
"lock->wait_lock" for all rt_mutexes, which is pretty useless when you
have to analyze a lockdep splat. Thanks for finding it nevertheless.

So the correct fix is:

Index: linux-2.6/kernel/rtmutex.c
===================================================================
--- linux-2.6.orig/kernel/rtmutex.c
+++ linux-2.6/kernel/rtmutex.c
@@ -1296,7 +1296,7 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init);
 void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
 				struct task_struct *proxy_owner)
 {
-	__rt_mutex_init(lock, NULL);
+	rt_mutex_init(lock);
 	debug_rt_mutex_proxy_lock(lock, proxy_owner);
 	rt_mutex_set_owner(lock, proxy_owner);
 	rt_mutex_deadlock_account_lock(lock, proxy_owner);

 
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