Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount

[Peter Zijlstra]

On Sun, May 08, 2016 at 09:56:08PM -0700, Davidlohr Bueso wrote:
> Read waiters are currently reference counted from the time it enters
> the slowpath until the lock is released and the waiter is awoken. This
> is fragile and superfluous considering everything occurs within down_read()
> without returning to the caller, and the very nature of the primitive does
> not suggest that the task can disappear from underneath us. In addition,
> spurious wakeups can make the whole refcount useless as get_task_struct()
> is only called when setting up the waiter.

So I think you're wrong here; imagine this:


        rwsem_down_read_failed()                        rwsem_wake()
          get_task_struct();
          raw_spin_lock_irq(&wait_lock);
          list_add_tail(&waiter.list, &wait_list);
          raw_spin_unlock_irq(&wait_lock);
                                                          
raw_spin_lock_irqsave(&wait_lock)
                                                          __rwsem_do_wake()
          while (true) {
            set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                                                            waiter->task = NULL
            if (!waiter.task) // true
              break;

          __set_task_state(tsk, TASK_RUNNING);

        do_exit();
                                                            
wake_up_process(tsk); /* BOOM */