rwsem: simplify rwsem_down_read_failed

When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS
adjustment doesn't cause other readers to block, so we never have to
worry about waking them back after canceling this adjustment in
rwsem_down_read_failed().

We also never want to steal the lock in rwsem_down_read_failed(), so we
don't have to grab the wait_lock either.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/rwsem.c b/lib/rwsem.c
index fb658af1c12c7f0e5ce56fbbb8c313a889b119eb..66f307e90761c0952335d16b7fe37a9b643c9fc8 100644 (file)
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -182,7 +182,6 @@ try_again_write:
  */
 struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 {
-       enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
        signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;
        struct rwsem_waiter waiter;
        struct task_struct *tsk = current;
@@ -190,7 +189,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 
        /* set up my own style of waitqueue */
        waiter.task = tsk;
-       waiter.type = type;
+       waiter.type = RWSEM_WAITING_FOR_READ;
        get_task_struct(tsk);
 
        raw_spin_lock_irq(&sem->wait_lock);
@@ -201,17 +200,9 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
        /* we're now waiting on the lock, but no longer actively locking */
        count = rwsem_atomic_update(adjustment, sem);
 
-       /* If there are no active locks, wake the front queued process(es) up.
-        *
-        * Alternatively, if we're called from a failed down_write(), there
-        * were already threads queued before us and there are no active
-        * writers, the lock must be read owned; so we try to wake any read
-        * locks that were queued ahead of us. */
+       /* 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);
-       else if (count > RWSEM_WAITING_BIAS &&
-                adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
-               sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 
        raw_spin_unlock_irq(&sem->wait_lock);
 
@@ -220,15 +211,6 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (!waiter.task)
                        break;
-
-               raw_spin_lock_irq(&sem->wait_lock);
-               /* Try to get the writer sem, may steal from the head writer: */
-               if (type == RWSEM_WAITING_FOR_WRITE)
-                       if (try_get_writer_sem(sem, &waiter)) {
-                               raw_spin_unlock_irq(&sem->wait_lock);
-                               return sem;
-                       }
-               raw_spin_unlock_irq(&sem->wait_lock);
                schedule();
        }