Re: [Qemu-devel] [PATCH uq/master 2/9] event_notifier: remove event_notifier_test

[Avi Kivity]

On 07/05/2012 06:16 PM, Paolo Bonzini wrote:
> This is broken; since the eventfd is used in nonblocking mode there
> is a race between reading and writing.
> 

> diff --git a/event_notifier.c b/event_notifier.c
> index 2b210f4..c339bfe 100644
> --- a/event_notifier.c
> +++ b/event_notifier.c
> @@ -51,18 +51,3 @@ int event_notifier_test_and_clear(EventNotifier *e)
>      int r = read(e->fd, &value, sizeof(value));
>      return r == sizeof(value);
>  }
> -
> -int event_notifier_test(EventNotifier *e)
> -{
> -    uint64_t value;
> -    int r = read(e->fd, &value, sizeof(value));
> -    if (r == sizeof(value)) {
> -        /* restore previous value. */
> -        int s = write(e->fd, &value, sizeof(value));
> -        /* never blocks because we use EFD_SEMAPHORE.
> -         * If we didn't we'd get EAGAIN on overflow
> -         * and we'd have to write code to ignore it. */
> -        assert(s == sizeof(value));
> -    }
> -    return r == sizeof(value);
> -}

I don't see the race.  Mind explaining?

It does however require than a poller be extra careful when reading; and
the function is silly anyway.

-- 
error compiling committee.c: too many arguments to function