[lttng-dev] [PATCH lttng-tools v4 6/8] Change lttng_poll_wait behaviour of compat-poll to match compat-epoll

[Jérémie Galarneau]

On Mon, Apr 01, 2019 at 01:59:23PM -0400, Yannick Lamarre wrote:
> This removes the need to verify for idle file descriptors and mitigates
> risks of bug due to behaviour mismatch.
> 
> Signed-off-by: Yannick Lamarre <ylamarre at efficios.com>
> ---
> 
> The assert in the for loop is removed.
> 
>  src/common/compat/compat-poll.c | 37 +++++++++++++++++++++++++++++++++----
>  1 file changed, 33 insertions(+), 4 deletions(-)
> 
> diff --git a/src/common/compat/compat-poll.c b/src/common/compat/compat-poll.c
> index be655728..5b752a28 100644
> --- a/src/common/compat/compat-poll.c
> +++ b/src/common/compat/compat-poll.c
> @@ -293,7 +293,8 @@ error:
>   */
>  int compat_poll_wait(struct lttng_poll_event *events, int timeout)
>  {
> -	int ret;
> +	int ret, active_fd_count;
> +	int idle_pfd_index = 0;
>  
>  	if (events == NULL || events->current.events == NULL) {
>  		ERR("poll wait arguments error");
> @@ -325,11 +326,39 @@ int compat_poll_wait(struct lttng_poll_event *events, int timeout)
>  		goto error;
>  	}
>  
> +	active_fd_count = ret;
> +
>  	/*
> -	 * poll() should always iterate on all FDs since we handle the pollset in
> -	 * user space and after poll returns, we have to try every fd for a match.
> +	 * Swap all active pollfd structs to the beginning of the
> +	 * array to emulate compat-epoll behaviour. This algorithm takes
> +	 * advantage of poll's returned value and the burst nature of active
> +	 * events on the file descriptors. The while loop guarantees that
> +	 * idle_pfd will always point to an idle fd.
>  	 */
> -	return events->wait.nb_fd;
> +	if (active_fd_count == events->wait.nb_fd) {
> +		goto end;
> +	}
> +	while (idle_pfd_index < active_fd_count &&
> +			events->wait.events[idle_pfd_index].revents != 0) {
> +		idle_pfd_index++;
> +	}
> +
> +	for (int i = idle_pfd_index + 1; idle_pfd_index < active_fd_count;
> +			i++) {

Declare 'i' at the beginning of the function.

Thanks!
Jérémie

> +		struct pollfd swap_pfd;
> +		struct pollfd *idle_pfd = &events->wait.events[idle_pfd_index];
> +		struct pollfd *current_pfd = &events->wait.events[i];
> +
> +		if (ipfd->revents != 0) {
> +			swap_pfd = *current_pfd;
> +			*current_pfd = *idle_pfd;
> +			*idle_pfd = swap_pfd;
> +			idle_pfd_index++;
> +		}
> +	}
> +
> +end:
> +	return ret;
>  
>  error:
>  	return -1;
> -- 
> 2.11.0
>