[PATCH 3/4] tty/ldsem: Add missing ACQUIRE to read_failed sleep loop

[Peter Zijlstra]

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'.

Cc: Peter Hurley <pe...@hurleysoftware.com>
Fixes: 4898e640caf0 ("tty: Add timed, writer-prioritized rw semaphore")
Reported-by: Will Deacon <w...@kernel.org>
Acked-by: Will Deacon <w...@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 drivers/tty/tty_ldsem.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -93,8 +93,7 @@ static void __ldsem_wake_readers(struct
 
        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 *se
        for (;;) {
                set_current_state(TASK_UNINTERRUPTIBLE);
 
-               if (!waiter.task)
+               if (!smp_load_acquire(&waiter.task))
                        break;
                if (!timeout)
                        break;