Subject: lglocks: Provide a RT safe variant
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 15 Jun 2011 11:02:21 +0200

lglocks by itself will spin in order to get the lock. This will end up
badly if a task with the highest priority keeps spinning while a task
with the lowest priority owns the lock.

Lets replace them with rt_mutex based locks so they can sleep, track
owner and boost if needed.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/lglock.h  |   18 +++++++++++++
 kernel/locking/lglock.c |   62 ++++++++++++++++++++++++++++++------------------
 2 files changed, 58 insertions(+), 22 deletions(-)

--- a/include/linux/lglock.h
+++ b/include/linux/lglock.h
@@ -34,13 +34,30 @@
 #endif
 
 struct lglock {
+#ifdef CONFIG_PREEMPT_RT_FULL
+	struct rt_mutex __percpu *lock;
+#else
 	arch_spinlock_t __percpu *lock;
+#endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lock_class_key lock_key;
 	struct lockdep_map    lock_dep_map;
 #endif
 };
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+# define DEFINE_LGLOCK(name)						\
+	static DEFINE_PER_CPU(struct rt_mutex, name ## _lock)		\
+	= __RT_MUTEX_INITIALIZER( name ## _lock);			\
+	struct lglock name = { .lock = &name ## _lock }
+
+# define DEFINE_STATIC_LGLOCK(name)					\
+	static DEFINE_PER_CPU(struct rt_mutex, name ## _lock)		\
+	= __RT_MUTEX_INITIALIZER( name ## _lock);			\
+	static struct lglock name = { .lock = &name ## _lock }
+
+#else
+
 #define DEFINE_LGLOCK(name)						\
 	static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock)		\
 	= __ARCH_SPIN_LOCK_UNLOCKED;					\
@@ -50,6 +67,7 @@ struct lglock {
 	static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock)		\
 	= __ARCH_SPIN_LOCK_UNLOCKED;					\
 	static struct lglock name = { .lock = &name ## _lock }
+#endif
 
 void lg_lock_init(struct lglock *lg, char *name);
 
--- a/kernel/locking/lglock.c
+++ b/kernel/locking/lglock.c
@@ -4,6 +4,15 @@
 #include <linux/cpu.h>
 #include <linux/string.h>
 
+#ifndef CONFIG_PREEMPT_RT_FULL
+# define lg_lock_ptr		arch_spinlock_t
+# define lg_do_lock(l)		arch_spin_lock(l)
+# define lg_do_unlock(l)	arch_spin_unlock(l)
+#else
+# define lg_lock_ptr		struct rt_mutex
+# define lg_do_lock(l)		__rt_spin_lock(l)
+# define lg_do_unlock(l)	__rt_spin_unlock(l)
+#endif
 /*
  * Note there is no uninit, so lglocks cannot be defined in
  * modules (but it's fine to use them from there)
@@ -12,51 +21,60 @@
 
 void lg_lock_init(struct lglock *lg, char *name)
 {
+#ifdef CONFIG_PREEMPT_RT_FULL
+	int i;
+
+	for_each_possible_cpu(i) {
+		struct rt_mutex *lock = per_cpu_ptr(lg->lock, i);
+
+		rt_mutex_init(lock);
+	}
+#endif
 	LOCKDEP_INIT_MAP(&lg->lock_dep_map, name, &lg->lock_key, 0);
 }
 EXPORT_SYMBOL(lg_lock_init);
 
 void lg_local_lock(struct lglock *lg)
 {
-	arch_spinlock_t *lock;
+	lg_lock_ptr *lock;
 
-	preempt_disable();
+	migrate_disable();
 	lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
 	lock = this_cpu_ptr(lg->lock);
-	arch_spin_lock(lock);
+	lg_do_lock(lock);
 }
 EXPORT_SYMBOL(lg_local_lock);
 
 void lg_local_unlock(struct lglock *lg)
 {
-	arch_spinlock_t *lock;
+	lg_lock_ptr *lock;
 
 	lock_release(&lg->lock_dep_map, 1, _RET_IP_);
 	lock = this_cpu_ptr(lg->lock);
-	arch_spin_unlock(lock);
-	preempt_enable();
+	lg_do_unlock(lock);
+	migrate_enable();
 }
 EXPORT_SYMBOL(lg_local_unlock);
 
 void lg_local_lock_cpu(struct lglock *lg, int cpu)
 {
-	arch_spinlock_t *lock;
+	lg_lock_ptr *lock;
 
-	preempt_disable();
+	preempt_disable_nort();
 	lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
 	lock = per_cpu_ptr(lg->lock, cpu);
-	arch_spin_lock(lock);
+	lg_do_lock(lock);
 }
 EXPORT_SYMBOL(lg_local_lock_cpu);
 
 void lg_local_unlock_cpu(struct lglock *lg, int cpu)
 {
-	arch_spinlock_t *lock;
+	lg_lock_ptr *lock;
 
 	lock_release(&lg->lock_dep_map, 1, _RET_IP_);
 	lock = per_cpu_ptr(lg->lock, cpu);
-	arch_spin_unlock(lock);
-	preempt_enable();
+	lg_do_unlock(lock);
+	preempt_enable_nort();
 }
 EXPORT_SYMBOL(lg_local_unlock_cpu);
 
@@ -70,15 +88,15 @@ void lg_double_lock(struct lglock *lg, i
 
 	preempt_disable();
 	lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
-	arch_spin_lock(per_cpu_ptr(lg->lock, cpu1));
-	arch_spin_lock(per_cpu_ptr(lg->lock, cpu2));
+	lg_do_lock(per_cpu_ptr(lg->lock, cpu1));
+	lg_do_lock(per_cpu_ptr(lg->lock, cpu2));
 }
 
 void lg_double_unlock(struct lglock *lg, int cpu1, int cpu2)
 {
 	lock_release(&lg->lock_dep_map, 1, _RET_IP_);
-	arch_spin_unlock(per_cpu_ptr(lg->lock, cpu1));
-	arch_spin_unlock(per_cpu_ptr(lg->lock, cpu2));
+	lg_do_unlock(per_cpu_ptr(lg->lock, cpu1));
+	lg_do_unlock(per_cpu_ptr(lg->lock, cpu2));
 	preempt_enable();
 }
 
@@ -86,12 +104,12 @@ void lg_global_lock(struct lglock *lg)
 {
 	int i;
 
-	preempt_disable();
+	preempt_disable_nort();
 	lock_acquire_exclusive(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
 	for_each_possible_cpu(i) {
-		arch_spinlock_t *lock;
+		lg_lock_ptr *lock;
 		lock = per_cpu_ptr(lg->lock, i);
-		arch_spin_lock(lock);
+		lg_do_lock(lock);
 	}
 }
 EXPORT_SYMBOL(lg_global_lock);
@@ -102,10 +120,10 @@ void lg_global_unlock(struct lglock *lg)
 
 	lock_release(&lg->lock_dep_map, 1, _RET_IP_);
 	for_each_possible_cpu(i) {
-		arch_spinlock_t *lock;
+		lg_lock_ptr *lock;
 		lock = per_cpu_ptr(lg->lock, i);
-		arch_spin_unlock(lock);
+		lg_do_unlock(lock);
 	}
-	preempt_enable();
+	preempt_enable_nort();
 }
 EXPORT_SYMBOL(lg_global_unlock);
