Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil
From: Peter Zijlstra
Date: Thu Nov 29 2018 - 12:27:13 EST
On Thu, Nov 29, 2018 at 12:02:19PM -0500, Waiman Long wrote:
> On 11/29/2018 11:06 AM, Peter Zijlstra wrote:
> > Why; at that point we know the wakeup will happen after, which is all we
> > require.
> >
> Thread 1 Thread 2 Thread 3
>
> rwsem_down_read_failed()
> raw_spin_lock_irq(&sem->wait_lock);
> list_add_tail(&waiter.list, &wait_list);
> raw_spin_unlock_irq(&sem->wait_lock);
> __rwsem_mark_wake();
> wake_q_add();
> wake_up_q();
> waiter->task =
> NULL; --+
> while (true)
> { |
>
> set_current_state(TASK_UNINTERRUPTIBLE);
> |
> if (!waiter.task) //
> false |
>
> break; |
>
> schedule();
> |
> }
> <-----+
> wake_up_q(&wake_q);
I think that thing is horribly whitespace damanaged. At least, it's not
making sense to me.
> OK, I got confused by the thread racing chart shown in the patch. It
> will be clearer if the clearing of waiter->task is moved down as shown.
> Otherwise, moving the clearing of waiter->task before wake_q_add() won't
> make a difference. So the patch can be a possible fix.
>
> Still we are talking about 3 threads racing with each other. The
> clearing of wake_q.next in wake_up_q() is not atomic and it is hard to
> predict the racing result of the concurrent wake_q operations between
> threads 2 and 3. The essence of my tentative patch is to prevent the
> concurrent wake_q operations in the first place.
wake_up_q() should, per the barriers in wake_up_process, ensure that if
wake_a_add() fails, there will be a wakeup of that task after that
point.
So if we put wake_up_q() at the location where wake_up_process() should
be, it should all work.
The bug in question is that it can happen at any time after
wake_q_add(), not necessarily at wake_up_q().