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[Qemu-devel] [PATCH] Check return values from g_poll and select
From: |
Fabien Chouteau |
Subject: |
[Qemu-devel] [PATCH] Check return values from g_poll and select |
Date: |
Tue, 8 Jan 2013 16:30:56 +0100 |
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>
---
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
--
1.7.9.5
- [Qemu-devel] [PATCH] Check return values from g_poll and select,
Fabien Chouteau <=