[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH] Check return values from g_poll and select
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] Check return values from g_poll and select |
Date: |
Wed, 09 Jan 2013 10:37:58 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
Il 08/01/2013 16:30, Fabien Chouteau ha scritto:
> The current implementation of os_host_main_loop_wait() on Windows,
> returns 1 only when a g_poll() event occurs because the return value of
> select() is overridden. This is wrong as we may skip a socket event, as
> shown in this example:
>
> 1. select() returns 0
> 2. g_poll() returns 1 (socket event occurs)
> 3. os_host_main_loop_wait() returns 1
> 4. qemu_iohandler_poll() sees no socket event because select() has
> return before the event occurs
> 5. select() returns 1
> 6. g_poll() returns 0 (g_poll overrides select's return value)
> 7. os_host_main_loop_wait() returns 0
> 8. qemu_iohandler_poll() doesn't check for socket events because the
> return value of os_host_main_loop_wait() is zero.
> 9. goto 5
>
> This patch use one variable for each of these return values, so we don't
> miss a select() event anymore.
>
> Also move the call to select() after g_poll(), this will improve latency
> as we don't have to go through two os_host_main_loop_wait() calls to
> detect a socket event.
>
> Signed-off-by: Fabien Chouteau <address@hidden>
Reviewed-by: Paolo Bonzini <address@hidden>
Paolo
> ---
> main-loop.c | 27 +++++++++++++--------------
> 1 file changed, 13 insertions(+), 14 deletions(-)
>
> diff --git a/main-loop.c b/main-loop.c
> index 54f38ae..6f52ac3 100644
> --- a/main-loop.c
> +++ b/main-loop.c
> @@ -330,7 +330,7 @@ void qemu_fd_register(int fd)
> static int os_host_main_loop_wait(uint32_t timeout)
> {
> GMainContext *context = g_main_context_default();
> - int ret, i;
> + int select_ret, g_poll_ret, ret, i;
> PollingEntry *pe;
> WaitObjects *w = &wait_objects;
> gint poll_timeout;
> @@ -345,13 +345,6 @@ static int os_host_main_loop_wait(uint32_t timeout)
> return ret;
> }
>
> - if (nfds >= 0) {
> - ret = select(nfds + 1, &rfds, &wfds, &xfds, &tv0);
> - if (ret != 0) {
> - timeout = 0;
> - }
> - }
> -
> g_main_context_prepare(context, &max_priority);
> n_poll_fds = g_main_context_query(context, max_priority, &poll_timeout,
> poll_fds, ARRAY_SIZE(poll_fds));
> @@ -367,9 +360,9 @@ static int os_host_main_loop_wait(uint32_t timeout)
> }
>
> qemu_mutex_unlock_iothread();
> - ret = g_poll(poll_fds, n_poll_fds + w->num, poll_timeout);
> + g_poll_ret = g_poll(poll_fds, n_poll_fds + w->num, poll_timeout);
> qemu_mutex_lock_iothread();
> - if (ret > 0) {
> + if (g_poll_ret > 0) {
> for (i = 0; i < w->num; i++) {
> w->revents[i] = poll_fds[n_poll_fds + i].revents;
> }
> @@ -384,12 +377,18 @@ static int os_host_main_loop_wait(uint32_t timeout)
> g_main_context_dispatch(context);
> }
>
> - /* If an edge-triggered socket event occurred, select will return a
> - * positive result on the next iteration. We do not need to do anything
> - * here.
> + /* Call select after g_poll to avoid a useless iteration and therefore
> + * improve socket latency.
> */
>
> - return ret;
> + if (nfds >= 0) {
> + select_ret = select(nfds + 1, &rfds, &wfds, &xfds, &tv0);
> + if (select_ret != 0) {
> + timeout = 0;
> + }
> + }
> +
> + return select_ret || g_poll_ret;
> }
> #endif
>
>