tty/ldsem, locking/rwsem: Add missing ACQUIRE to read_failed sleep loop

While reviewing rwsem down_slowpath, Will noticed ldsem had a copy of
a bug we just found for rwsem.

  X = 0;

  CPU0			CPU1

  rwsem_down_read()
    for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);

                        X = 1;
                        rwsem_up_write();
                          rwsem_mark_wake()
                            atomic_long_add(adjustment, &sem->count);
                            smp_store_release(&waiter->task, NULL);

      if (!waiter.task)
        break;

      ...
    }

  r = X;

Allows 'r == 0'.

Reported-by: Will Deacon <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Will Deacon <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Fixes: 4898e64 ("tty: Add timed, writer-prioritized rw semaphore")
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

drivers/tty/tty_ldsem.c

@@ -93,8 +93,7 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)
 
 	list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
 		tsk = waiter->task;
-		smp_mb();
-		waiter->task = NULL;
+		smp_store_release(&waiter->task, NULL);
 		wake_up_process(tsk);
 		put_task_struct(tsk);
 	}
@@ -194,7 +193,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
 
-		if (!waiter.task)
+		if (!smp_load_acquire(&waiter.task))
 			break;
 		if (!timeout)
 			break;