1 files changed, 32 insertions, 32 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 9a675fa9d78e..bbe48c04f363 100644
--- a/lib/rwsem.c
+++ b/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);