[PATCH] event-loop: Only read one epoll event per loop dispatch.

[Andreas Ericsson]

On 03/17/2012 09:39 AM, Jonas Ådahl wrote:
> In order for users of the event loop to manipulate the state of it or
> others, only one event may be processed per epoll_wait. When multiple
> events are queued up and a user removes an event input source from an
> event loop from within a dispatch, if the removed source was one of the
> queued up events the program may crash with a segmentation faults.
> 

This patch will kill performance completely. I've done this exact
same thing in an I/O-broker library I wrote, and it's disastrous
for any kind of throughput.

My testcase served several thousand events per second though from
about 500 filedescriptors, so this might not be much of an issue
for Wayland, which I suspect will have fewer fd's and fewer events
per second.


> In order to allow queuing up multiple events there needs to be a way to
> manipulate the state without destroying the source event.
> 
> Signed-off-by: Jonas Ådahl<jadahl at gmail.com>
> ---
>   src/event-loop.c |   20 ++++++++++----------
>   1 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/src/event-loop.c b/src/event-loop.c
> index 2dfe0ae..cc67383 100644
> --- a/src/event-loop.c
> +++ b/src/event-loop.c
> @@ -447,23 +447,23 @@ dispatch_idle_sources(struct wl_event_loop *loop)
>   WL_EXPORT int
>   wl_event_loop_dispatch(struct wl_event_loop *loop, int timeout)
>   {
> -	struct epoll_event ep[32];
> +	struct epoll_event ep;
>   	struct wl_event_source *source;
> -	int i, count, n;
> +	int count, n;
> 
>   	dispatch_idle_sources(loop);
> 
> -	count = epoll_wait(loop->epoll_fd, ep, ARRAY_LENGTH(ep), timeout);
> +	count = epoll_wait(loop->epoll_fd,&ep, 1, timeout);
>   	if (count<  0)
>   		return -1;
> -	n = 0;
> -	for (i = 0; i<  count; i++) {
> -		source = ep[i].data.ptr;


How about just

		if (!source)
			continue;

here? In particular since removing a source *must* remove it from
the event handling for it not to crash out later.

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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