1 files changed, 58 insertions, 32 deletions

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 24df4d98f7d2..e66f37ebc6f6 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -72,7 +72,7 @@
 		#c, atomic_long_read(&(sem)->count),		\
 		(unsigned long) sem->magic,			\
 		atomic_long_read(&(sem)->owner), (long)current,	\
-		list_empty(&(sem)->wait_list) ? "" : "not "))	\
+		(sem)->first_waiter ? "" : "not "))		\
 			debug_locks_off();			\
 	} while (0)
 #else
@@ -321,7 +321,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
 #endif
 	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
 	raw_spin_lock_init(&sem->wait_lock);
-	INIT_LIST_HEAD(&sem->wait_list);
+	sem->first_waiter = NULL;
 	atomic_long_set(&sem->owner, 0L);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	osq_lock_init(&sem->osq);
@@ -341,8 +341,6 @@ struct rwsem_waiter {
 	unsigned long timeout;
 	bool handoff_set;
 };
-#define rwsem_first_waiter(sem) \
-	list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
 
 enum rwsem_wake_type {
 	RWSEM_WAKE_ANY,		/* Wake whatever's at head of wait list */
@@ -365,12 +363,21 @@ enum rwsem_wake_type {
  */
 #define MAX_READERS_WAKEUP	0x100
 
-static inline void
-rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
+static inline
+bool __rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
 {
-	lockdep_assert_held(&sem->wait_lock);
-	list_add_tail(&waiter->list, &sem->wait_list);
-	/* caller will set RWSEM_FLAG_WAITERS */
+	if (list_empty(&waiter->list)) {
+		sem->first_waiter = NULL;
+		return true;
+	}
+
+	if (sem->first_waiter == waiter) {
+		sem->first_waiter = list_first_entry(&waiter->list,
+						     struct rwsem_waiter, list);
+	}
+	list_del(&waiter->list);
+
+	return false;
 }
 
 /*
@@ -385,14 +392,23 @@ static inline bool
 rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
 {
 	lockdep_assert_held(&sem->wait_lock);
-	list_del(&waiter->list);
-	if (likely(!list_empty(&sem->wait_list)))
+	if (__rwsem_del_waiter(sem, waiter))
 		return true;
-
 	atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
 	return false;
 }
 
+static inline
+struct rwsem_waiter *next_waiter(const struct rw_semaphore *sem,
+				 const struct rwsem_waiter *waiter)
+{
+	struct rwsem_waiter *next = list_first_entry(&waiter->list,
+						     struct rwsem_waiter, list);
+	if (next == sem->first_waiter)
+		return NULL;
+	return next;
+}
+
 /*
  * handle the lock release when processes blocked on it that can now run
  * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
@@ -411,7 +427,7 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
 			    enum rwsem_wake_type wake_type,
 			    struct wake_q_head *wake_q)
 {
-	struct rwsem_waiter *waiter, *tmp;
+	struct rwsem_waiter *waiter, *next;
 	long oldcount, woken = 0, adjustment = 0;
 	struct list_head wlist;
 
@@ -421,7 +437,7 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
 	 * Take a peek at the queue head waiter such that we can determine
 	 * the wakeup(s) to perform.
 	 */
-	waiter = rwsem_first_waiter(sem);
+	waiter = sem->first_waiter;
 
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
 		if (wake_type == RWSEM_WAKE_ANY) {
@@ -506,25 +522,28 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
 	 *    put them into wake_q to be woken up later.
 	 */
 	INIT_LIST_HEAD(&wlist);
-	list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
+	do {
+		next = next_waiter(sem, waiter);
 		if (waiter->type == RWSEM_WAITING_FOR_WRITE)
 			continue;
 
 		woken++;
 		list_move_tail(&waiter->list, &wlist);
+		if (sem->first_waiter == waiter)
+			sem->first_waiter = next;
 
 		/*
 		 * Limit # of readers that can be woken up per wakeup call.
 		 */
 		if (unlikely(woken >= MAX_READERS_WAKEUP))
 			break;
-	}
+	} while ((waiter = next) != NULL);
 
 	adjustment = woken * RWSEM_READER_BIAS - adjustment;
 	lockevent_cond_inc(rwsem_wake_reader, woken);
 
 	oldcount = atomic_long_read(&sem->count);
-	if (list_empty(&sem->wait_list)) {
+	if (!sem->first_waiter) {
 		/*
 		 * Combined with list_move_tail() above, this implies
 		 * rwsem_del_waiter().
@@ -545,7 +564,7 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
 		atomic_long_add(adjustment, &sem->count);
 
 	/* 2nd pass */
-	list_for_each_entry_safe(waiter, tmp, &wlist, list) {
+	list_for_each_entry_safe(waiter, next, &wlist, list) {
 		struct task_struct *tsk;
 
 		tsk = waiter->task;
@@ -577,7 +596,7 @@ rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
 		      struct wake_q_head *wake_q)
 		      __releases(&sem->wait_lock)
 {
-	bool first = rwsem_first_waiter(sem) == waiter;
+	bool first = sem->first_waiter == waiter;
 
 	wake_q_init(wake_q);
 
@@ -603,7 +622,7 @@ rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
 static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 					struct rwsem_waiter *waiter)
 {
-	struct rwsem_waiter *first = rwsem_first_waiter(sem);
+	struct rwsem_waiter *first = sem->first_waiter;
 	long count, new;
 
 	lockdep_assert_held(&sem->wait_lock);
@@ -639,7 +658,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 			new |= RWSEM_WRITER_LOCKED;
 			new &= ~RWSEM_FLAG_HANDOFF;
 
-			if (list_is_singular(&sem->wait_list))
+			if (list_empty(&first->list))
 				new &= ~RWSEM_FLAG_WAITERS;
 		}
 	} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
@@ -659,7 +678,8 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 	 * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
 	 * success.
 	 */
-	list_del(&waiter->list);
+	__rwsem_del_waiter(sem, waiter);
+
 	rwsem_set_owner(sem);
 	return true;
 }
@@ -994,7 +1014,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 {
 	long adjustment = -RWSEM_READER_BIAS;
 	long rcnt = (count >> RWSEM_READER_SHIFT);
-	struct rwsem_waiter waiter;
+	struct rwsem_waiter waiter, *first;
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
@@ -1019,7 +1039,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 		 */
 		if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
 			raw_spin_lock_irq(&sem->wait_lock);
-			if (!list_empty(&sem->wait_list))
+			if (sem->first_waiter)
 				rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
 						&wake_q);
 			raw_spin_unlock_irq(&sem->wait_lock);
@@ -1035,7 +1055,8 @@ queue:
 	waiter.handoff_set = false;
 
 	raw_spin_lock_irq(&sem->wait_lock);
-	if (list_empty(&sem->wait_list)) {
+	first = sem->first_waiter;
+	if (!first) {
 		/*
 		 * In case the wait queue is empty and the lock isn't owned
 		 * by a writer, this reader can exit the slowpath and return
@@ -1051,8 +1072,11 @@ queue:
 			return sem;
 		}
 		adjustment += RWSEM_FLAG_WAITERS;
+		INIT_LIST_HEAD(&waiter.list);
+		sem->first_waiter = &waiter;
+	} else {
+		list_add_tail(&waiter.list, &first->list);
 	}
-	rwsem_add_waiter(sem, &waiter);
 
 	/* we're now waiting on the lock, but no longer actively locking */
 	count = atomic_long_add_return(adjustment, &sem->count);
@@ -1110,7 +1134,7 @@ out_nolock:
 static struct rw_semaphore __sched *
 rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 {
-	struct rwsem_waiter waiter;
+	struct rwsem_waiter waiter, *first;
 	DEFINE_WAKE_Q(wake_q);
 
 	/* do optimistic spinning and steal lock if possible */
@@ -1129,10 +1153,10 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 	waiter.handoff_set = false;
 
 	raw_spin_lock_irq(&sem->wait_lock);
-	rwsem_add_waiter(sem, &waiter);
 
-	/* we're now waiting on the lock */
-	if (rwsem_first_waiter(sem) != &waiter) {
+	first = sem->first_waiter;
+	if (first) {
+		list_add_tail(&waiter.list, &first->list);
 		rwsem_cond_wake_waiter(sem, atomic_long_read(&sem->count),
 				       &wake_q);
 		if (!wake_q_empty(&wake_q)) {
@@ -1145,6 +1169,8 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 			raw_spin_lock_irq(&sem->wait_lock);
 		}
 	} else {
+		INIT_LIST_HEAD(&waiter.list);
+		sem->first_waiter = &waiter;
 		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
 	}
 
@@ -1218,7 +1244,7 @@ static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 
 	raw_spin_lock_irqsave(&sem->wait_lock, flags);
 
-	if (!list_empty(&sem->wait_list))
+	if (sem->first_waiter)
 		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
 
 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
@@ -1239,7 +1265,7 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 
 	raw_spin_lock_irqsave(&sem->wait_lock, flags);
 
-	if (!list_empty(&sem->wait_list))
+	if (sem->first_waiter)
 		rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
 
 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);