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 <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 4898e640caf0 ("tty: Add timed, writer-prioritized rw semaphore")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 717292c1c0dfc352fc0e71d836fe900332ae09cd..60ff236a3d63df45bea93d32239e29e0ca8b80ec 100644 (file)
--- a/drivers/tty/tty_ldsem.c
+++ b/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;