---

yaml
---
r: 373755
b: refs/heads/master
c: fe6e674
h: refs/heads/master
i:
  373753: 514a0b3
  373751: 933598b
v: v3

[refs]

@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 8cf5322ce69afea1fab6a6270db24d057d664798
+refs/heads/master: fe6e674c6187d4f452a679ced7e95262bd517936

trunk/lib/rwsem.c

@@ -4,6 +4,7 @@
  * Derived from arch/i386/kernel/semaphore.c
  *
  * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
+ * and Michel Lespinasse <walken@google.com>
  */
 #include <linux/rwsem.h>
 #include <linux/sched.h>
@@ -41,13 +42,11 @@ struct rwsem_waiter {
 	enum rwsem_waiter_type type;
 };
 
-/* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
- * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
- * since the rwsem value was observed.
- */
-#define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
-#define RWSEM_WAKE_NO_ACTIVE  1 /* rwsem was observed with no active thread */
-#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
+enum rwsem_wake_type {
+	RWSEM_WAKE_ANY,		/* Wake whatever's at head of wait list */
+	RWSEM_WAKE_READERS,	/* Wake readers only */
+	RWSEM_WAKE_READ_OWNED	/* Waker thread holds the read lock */
+};
 
 /*
  * handle the lock release when processes blocked on it that can now run
@@ -60,16 +59,16 @@ struct rwsem_waiter {
  * - writers are only woken if downgrading is false
  */
 static struct rw_semaphore *
-__rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
+__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 {
 	struct rwsem_waiter *waiter;
 	struct task_struct *tsk;
 	struct list_head *next;
-	signed long woken, loop, adjustment;
+	signed long oldcount, woken, loop, adjustment;
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
-		if (wake_type != RWSEM_WAKE_READ_OWNED)
+		if (wake_type == RWSEM_WAKE_ANY)
 			/* Wake writer at the front of the queue, but do not
 			 * grant it the lock yet as we want other writers
 			 * to be able to steal it.  Readers, on the other hand,
@@ -79,24 +78,24 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 		goto out;
 	}
 
-	/* If we come here from up_xxxx(), another thread might have reached
-	 * rwsem_down_failed_common() before we acquired the spinlock and
-	 * woken up a waiter, making it now active.  We prefer to check for
-	 * this first in order to not spend too much time with the spinlock
-	 * held if we're not going to be able to wake up readers in the end.
-	 *
-	 * Note that we do not need to update the rwsem count: any writer
-	 * trying to acquire rwsem will run rwsem_down_write_failed() due
-	 * to the waiting threads and block trying to acquire the spinlock.
-	 *
-	 * We use a dummy atomic update in order to acquire the cache line
-	 * exclusively since we expect to succeed and run the final rwsem
-	 * count adjustment pretty soon.
+	/* Writers might steal the lock before we grant it to the next reader.
+	 * We prefer to do the first reader grant before counting readers
+	 * so we can bail out early if a writer stole the lock.
 	 */
-	if (wake_type == RWSEM_WAKE_ANY &&
-	    rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
-		/* Someone grabbed the sem for write already */
-		goto out;
+	adjustment = 0;
+	if (wake_type != RWSEM_WAKE_READ_OWNED) {
+		adjustment = RWSEM_ACTIVE_READ_BIAS;
+ try_reader_grant:
+		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
+		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
+			/* A writer stole the lock. Undo our reader grant. */
+			if (rwsem_atomic_update(-adjustment, sem) &
+						RWSEM_ACTIVE_MASK)
+				goto out;
+			/* Last active locker left. Retry waking readers. */
+			goto try_reader_grant;
+		}
+	}
 
 	/* Grant an infinite number of read locks to the readers at the front
 	 * of the queue.  Note we increment the 'active part' of the count by
@@ -114,12 +113,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 
 	} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
 
-	adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
+	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
 	if (waiter->type != RWSEM_WAITING_FOR_WRITE)
 		/* hit end of list above */
 		adjustment -= RWSEM_WAITING_BIAS;
 
-	rwsem_atomic_add(adjustment, sem);
+	if (adjustment)
+		rwsem_atomic_add(adjustment, sem);
 
 	next = sem->wait_list.next;
 	loop = woken;
@@ -164,8 +164,8 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	count = rwsem_atomic_update(adjustment, sem);
 
 	/* If there are no active locks, wake the front queued process(es). */
-	if (count == RWSEM_WAITING_BIAS)
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
+	if (!(count & RWSEM_ACTIVE_MASK))
+		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
 
 	raw_spin_unlock_irq(&sem->wait_lock);
 
@@ -209,7 +209,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
 	 * any read locks that were queued ahead of us. */
 	if (count > RWSEM_WAITING_BIAS &&
 	    adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
+		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
 
 	/* wait until we successfully acquire the lock */
 	set_task_state(tsk, TASK_UNINTERRUPTIBLE);