use a mutex for percpu rwsemaphores

bcachefs is using a percpu rwsem to protect percpu data structures, and
in userspace we don't have real percpu data structures - so we need to
guard all access to them with a mutex.

include/linux/percpu-rwsem.h

@@ -7,63 +7,44 @@
 #include <linux/preempt.h>
 
 struct percpu_rw_semaphore {
-	pthread_rwlock_t	lock;
+	pthread_mutex_t		lock;
 };
 
-#define DEFINE_STATIC_PERCPU_RWSEM(name)				\
-static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name);		\
-static struct percpu_rw_semaphore name = {				\
-	.rss = __RCU_SYNC_INITIALIZER(name.rss, RCU_SCHED_SYNC),	\
-	.read_count = &__percpu_rwsem_rc_##name,			\
-	.rw_sem = __RWSEM_INITIALIZER(name.rw_sem),			\
-	.writer = __RCUWAIT_INITIALIZER(name.writer),			\
-}
-
-extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
-extern void __percpu_up_read(struct percpu_rw_semaphore *);
-
 static inline void percpu_down_read_preempt_disable(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_rdlock(&sem->lock);
+	pthread_mutex_lock(&sem->lock);
-	preempt_disable();
 }
 
 static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_rdlock(&sem->lock);
+	pthread_mutex_lock(&sem->lock);
-}
-
-static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
-{
-	return !pthread_rwlock_tryrdlock(&sem->lock);
 }
 
 static inline void percpu_up_read_preempt_enable(struct percpu_rw_semaphore *sem)
 {
-	preempt_enable();
+	pthread_mutex_unlock(&sem->lock);
-	pthread_rwlock_unlock(&sem->lock);
 }
 
 static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_unlock(&sem->lock);
+	pthread_mutex_unlock(&sem->lock);
 }
 
 static inline void percpu_down_write(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_wrlock(&sem->lock);
+	pthread_mutex_lock(&sem->lock);
 }
 
 static inline void percpu_up_write(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_unlock(&sem->lock);
+	pthread_mutex_unlock(&sem->lock);
 }
 
 static inline void percpu_free_rwsem(struct percpu_rw_semaphore *sem) {}
 
 static inline int percpu_init_rwsem(struct percpu_rw_semaphore *sem)
 {
-	pthread_rwlock_init(&sem->lock, NULL);
+	pthread_mutex_init(&sem->lock, NULL);
 	return 0;
 }