read_lock migrate_disable pushdown to rt_read_lock

pushdown of migrate_disable/enable from read_*lock* to the rt_read_*lock*
api level

general mapping to mutexes:

read_*lock*
  `-> rt_read_*lock*
          `-> __spin_lock (the sleeping spin locks)
                 `-> rt_mutex

The real read_lock* mapping:

          read_lock_irqsave -.
read_lock_irq                `-> rt_read_lock_irqsave()
       `->read_lock ---------.       \
          read_lock_bh ------+        \
                             `--> rt_read_lock()
                                   if (rt_mutex_owner(lock) != current){
                                           `-> __rt_spin_lock()
                                                rt_spin_lock_fastlock()
                                                       `->rt_mutex_cmpxchg()
                                    migrate_disable()
                                   }
                                   rwlock->read_depth++;
read_trylock mapping:

read_trylock
          `-> rt_read_trylock
               if (rt_mutex_owner(lock) != current){
                                              `-> rt_mutex_trylock()
                                                   rt_mutex_fasttrylock()
                                                    rt_mutex_cmpxchg()
                migrate_disable()
               }
               rwlock->read_depth++;

read_unlock* mapping:

read_unlock_bh --------+
read_unlock_irq -------+
read_unlock_irqrestore +
read_unlock -----------+
                       `-> rt_read_unlock()
                            if(--rwlock->read_depth==0){
                                      `-> __rt_spin_unlock()
                                           rt_spin_lock_fastunlock()
                                                        `-> rt_mutex_cmpxchg()
                             migrate_disable()
                            }

So calls to migrate_disable/enable() are better placed at the rt_read_*
level of lock/trylock/unlock as all of the read_*lock* API has this as a
common path. In the rt_read* API of lock/trylock/unlock the nesting level
is already being recorded in rwlock->read_depth, so we can push down the
migrate disable/enable to that level and condition it on the read_depth
going from 0 to 1 -> migrate_disable and 1 to 0 -> migrate_enable. This
eliminates the recursive calls that were needed when migrate_disable/enable
was done at the read_*lock* level.

The approach to read_*_bh also eliminates the concerns raised with the
regards to api inbalances (read_lock_bh -> read_unlock+local_bh_enable)

Tested-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

include/linux/rwlock_rt.h

@@ -33,7 +33,6 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
 #define read_lock_irqsave(lock, flags)			\
 	do {						\
 		typecheck(unsigned long, flags);	\
-		migrate_disable();			\
 		flags = rt_read_lock_irqsave(lock);	\
 	} while (0)
 
@@ -45,14 +44,12 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
 
 #define read_lock(lock)					\
 	do {						\
-		migrate_disable();			\
 		rt_read_lock(lock);			\
 	} while (0)
 
 #define read_lock_bh(lock)				\
 	do {						\
 		local_bh_disable();			\
-		migrate_disable();			\
 		rt_read_lock(lock);			\
 	} while (0)
 
@@ -74,13 +71,11 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
 #define read_unlock(lock)				\
 	do {						\
 		rt_read_unlock(lock);			\
-		migrate_enable();			\
 	} while (0)
 
 #define read_unlock_bh(lock)				\
 	do {						\
 		rt_read_unlock(lock);			\
-		migrate_enable();			\
 		local_bh_enable();			\
 	} while (0)
 
@@ -104,7 +99,6 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
 		typecheck(unsigned long, flags);	\
 		(void) flags;				\
 		rt_read_unlock(lock);			\
-		migrate_enable();			\
 	} while (0)
 
 #define write_unlock_irqrestore(lock, flags) \

kernel/locking/rt.c

@@ -211,17 +211,19 @@ int __lockfunc rt_read_trylock(rwlock_t *rwlock)
 	 * but not when read_depth == 0 which means that the lock is
 	 * write locked.
 	 */
-	migrate_disable();
-	if (rt_mutex_owner(lock) != current)
+	if (rt_mutex_owner(lock) != current) {
 		ret = rt_mutex_trylock(lock);
-	else if (!rwlock->read_depth)
+		if (ret)
+			migrate_disable();
+
+	} else if (!rwlock->read_depth) {
 		ret = 0;
+	}
 
 	if (ret) {
 		rwlock->read_depth++;
 		rwlock_acquire(&rwlock->dep_map, 0, 1, _RET_IP_);
-	} else
-		migrate_enable();
+	}
 
 	return ret;
 }
@@ -244,8 +246,10 @@ void __lockfunc rt_read_lock(rwlock_t *rwlock)
 	/*
 	 * recursive read locks succeed when current owns the lock
 	 */
-	if (rt_mutex_owner(lock) != current)
+	if (rt_mutex_owner(lock) != current) {
 		__rt_spin_lock(lock);
+		migrate_disable();
+	}
 	rwlock->read_depth++;
 }
 
@@ -265,8 +269,10 @@ void __lockfunc rt_read_unlock(rwlock_t *rwlock)
 	rwlock_release(&rwlock->dep_map, 1, _RET_IP_);
 
 	/* Release the lock only when read_depth is down to 0 */
-	if (--rwlock->read_depth == 0)
+	if (--rwlock->read_depth == 0) {
 		__rt_spin_unlock(&rwlock->lock);
+		migrate_enable();
+	}
 }
 EXPORT_SYMBOL(rt_read_unlock);