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Check return values from g_poll and select

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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.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Fabien Chouteau 2013-01-08 16:30:56 +01:00 committed by Anthony Liguori
parent 7cd5da7eef
commit 5e3bc735d9
1 changed files with 13 additions and 14 deletions
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27
main-loop.c
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@ -330,7 +330,7 @@ void qemu_fd_register(int fd)
static int os_host_main_loop_wait(uint32_t timeout) static int os_host_main_loop_wait(uint32_t timeout)
{ {
GMainContext *context = g_main_context_default(); GMainContext *context = g_main_context_default();
int ret, i; int select_ret, g_poll_ret, ret, i;
PollingEntry *pe; PollingEntry *pe;
WaitObjects *w = &wait_objects; WaitObjects *w = &wait_objects;
gint poll_timeout; gint poll_timeout;
@ -345,13 +345,6 @@ static int os_host_main_loop_wait(uint32_t timeout)
return ret; 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); g_main_context_prepare(context, &max_priority);
n_poll_fds = g_main_context_query(context, max_priority, &poll_timeout, n_poll_fds = g_main_context_query(context, max_priority, &poll_timeout,
poll_fds, ARRAY_SIZE(poll_fds)); poll_fds, ARRAY_SIZE(poll_fds));
@ -367,9 +360,9 @@ static int os_host_main_loop_wait(uint32_t timeout)
} }
qemu_mutex_unlock_iothread(); 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(); qemu_mutex_lock_iothread();
if (ret > 0) { if (g_poll_ret > 0) {
for (i = 0; i < w->num; i++) { for (i = 0; i < w->num; i++) {
w->revents[i] = poll_fds[n_poll_fds + i].revents; 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); g_main_context_dispatch(context);
} }
/* If an edge-triggered socket event occurred, select will return a /* Call select after g_poll to avoid a useless iteration and therefore
* positive result on the next iteration. We do not need to do anything * improve socket latency.
* here.
*/ */
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 #endif