locking/rt: Take RCU nesting into account for might_sleep()

The RT patches contained a cheap hack to ignore the RCU nesting depth in
might_sleep() checks, which was a pragmatic but incorrect workaround.

The general rule that rcu_read_lock() held sections cannot voluntary sleep
does apply even on RT kernels. Though the substitution of spin/rw locks on
RT enabled kernels has to be exempt from that rule. On !RT a spin_lock()
can obviously nest inside a rcu read side critical section as the lock
acquisition is not going to block, but on RT this is not longer the case
due to the 'sleeping' spin lock substitution.

Instead of generally ignoring the RCU nesting depth in might_sleep()
checks, pass the rcu_preempt_depth() as offset argument to might_sleep()
from spin/read/write_lock() which makes the check work correctly even in
RCU read side critical sections.

The actual blocking on such a substituted lock within a RCU read side
critical section is already handled correctly in __schedule() by treating
it as a "preemption" of the RCU read side critical section.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/locking/spinlock_rt.c

@@ -24,6 +24,14 @@
 #define RT_MUTEX_BUILD_SPINLOCKS
 #include "rtmutex.c"
 
+/*
+ * Use ___might_sleep() which skips the state check and take RCU nesting
+ * into account as spin/read/write_lock() can legitimately nest into an RCU
+ * read side critical section:
+ */
+#define rtlock_might_sleep()					\
+	___might_sleep(__FILE__, __LINE__, rcu_preempt_depth())
+
 static __always_inline void rtlock_lock(struct rt_mutex_base *rtm)
 {
 	if (unlikely(!rt_mutex_cmpxchg_acquire(rtm, NULL, current)))
@@ -32,7 +40,7 @@ static __always_inline void rtlock_lock(struct rt_mutex_base *rtm)
 
 static __always_inline void __rt_spin_lock(spinlock_t *lock)
 {
-	___might_sleep(__FILE__, __LINE__, 0);
+	rtlock_might_sleep();
 	rtlock_lock(&lock->lock);
 	rcu_read_lock();
 	migrate_disable();
@@ -210,7 +218,7 @@ EXPORT_SYMBOL(rt_write_trylock);
 
 void __sched rt_read_lock(rwlock_t *rwlock)
 {
-	___might_sleep(__FILE__, __LINE__, 0);
+	rtlock_might_sleep();
 	rwlock_acquire_read(&rwlock->dep_map, 0, 0, _RET_IP_);
 	rwbase_read_lock(&rwlock->rwbase, TASK_RTLOCK_WAIT);
 	rcu_read_lock();
@@ -220,7 +228,7 @@ EXPORT_SYMBOL(rt_read_lock);
 
 void __sched rt_write_lock(rwlock_t *rwlock)
 {
-	___might_sleep(__FILE__, __LINE__, 0);
+	rtlock_might_sleep();
 	rwlock_acquire(&rwlock->dep_map, 0, 0, _RET_IP_);
 	rwbase_write_lock(&rwlock->rwbase, TASK_RTLOCK_WAIT);
 	rcu_read_lock();