Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

Manfred Spraul
2011-09-17T12:25:47Z

On 09/16/2011 02:18 PM, Peter Zijlstra wrote:
> On Thu, 2011-09-15 at 19:29 +0200, Manfred Spraul wrote:
>> What is broken?
> So basically sembench was broken and the futex patch is causing spurious
> wakeups.
>
> I've got the below patch to fix up the sem code.
>
> One more question, do the sem wakeups need to be issued in FIFO order?
> There's a comment in there:
>
> * User space visible behavior:
> * - FIFO ordering for semop() operations (just FIFO, not starvation
> * protection)
>
> that seems to suggest the sem ops processing is in FIFO order, but does
> the user visible effect propagate to the wakeup order?
I'd ask the question the other way arould:
Is the wakeup order user visible?
IMHO: No, the scheduler might reorder the tasks anyway.

>
> /*
> * If an interrupt occurred we have to clean up the queue
> */
> if (timeout&& jiffies_left == 0)
> error = -EAGAIN;
> +
> + if (error == -EINTR&& !signal_pending(current))
> + goto retry;
> +
> unlink_queue(sma,&queue);
>
> out_unlock_free:
Good solution.
-ERESTARTNOHAND would be even better, but this is definitively better
than the current code.

--
Manfred