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>

1 files changed, 2 insertions, 20 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index fb658af1c12c..66f307e90761 100644
--- 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();
         }