[Qemu-devel] Re: [PATCH] Fix alarm_timer race with select

[Jan Kiszka <jan.kiszka@web.de>]

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Anthony Liguori wrote:
> Anthony Liguori wrote:
>> Jan Kiszka wrote:
>>
>> Perhaps we should move the alarm timer check rearming out of the main
>> loop and into a qemu_set_fd_handler2() handler?
> 
> And now that I think about it, I see no reason why the timer expiration
> checks couldn't be moved to the same handler.  I'd have to look more
> closely at how that would interact with icount though.

I cannot yet follow you here. But I my impression is that this will be
an additional change that should come in a separate patch, no?

Find the O_NONBLOCK remark addressed below.

Jan

------------>

Changing the default IO timeout to 5 s (#5578) made a race visible
between the alarm_timer and select() in main_loop_wait(): If the timer
fired before select was able to block, the full select() timeout could
have been applied instead of returning immediately. Since #5578, this
causes heavy problems to the Musicpal board emulation with stalls up to
5 s.

The following patch introduces a pipe that is written to by
host_alarm_handler and select()'ed in main_loop_wait(). This avoids
prevents that select() blocks though a timer has fired and waits for
processing.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
---
 vl.c |   21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

Index: b/vl.c
===================================================================
--- a/vl.c
+++ b/vl.c
@@ -884,6 +884,7 @@ static void qemu_rearm_alarm_timer(struc
 #define MIN_TIMER_REARM_US 250
 
 static struct qemu_alarm_timer *alarm_timer;
+static int alarm_timer_rfd, alarm_timer_wfd;
 
 #ifdef _WIN32
 
@@ -1303,12 +1304,15 @@ static void host_alarm_handler(int host_
                                qemu_get_clock(vm_clock))) ||
         qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
                            qemu_get_clock(rt_clock))) {
+        CPUState *env = next_cpu;
+        char byte = 0;
+
 #ifdef _WIN32
         struct qemu_alarm_win32 *data = ((struct qemu_alarm_timer*)dwUser)->priv;
         SetEvent(data->host_alarm);
 #endif
-        CPUState *env = next_cpu;
 
+        write(alarm_timer_wfd, &byte, sizeof(byte));
         alarm_timer->flags |= ALARM_FLAG_EXPIRED;
 
         if (env) {
@@ -1673,6 +1677,15 @@ static void init_timer_alarm(void)
 {
     struct qemu_alarm_timer *t = NULL;
     int i, err = -1;
+    int fds[2];
+
+    if (pipe(fds) || fcntl(fds[0], F_SETFL, O_NONBLOCK)
+        || fcntl(fds[1], F_SETFL, O_NONBLOCK)) {
+        perror("creating timer pipe");
+        exit(1);
+    }
+    alarm_timer_rfd = fds[0];
+    alarm_timer_wfd = fds[1];
 
     for (i = 0; alarm_timers[i].name; i++) {
         t = &alarm_timers[i];
@@ -4427,6 +4440,7 @@ void main_loop_wait(int timeout)
     /* XXX: separate device handlers from system ones */
     nfds = -1;
     FD_ZERO(&rfds);
+    FD_SET(alarm_timer_rfd, &rfds);
     FD_ZERO(&wfds);
     FD_ZERO(&xfds);
     for(ioh = first_io_handler; ioh != NULL; ioh = ioh->next) {
@@ -4500,6 +4514,11 @@ void main_loop_wait(int timeout)
                     qemu_get_clock(rt_clock));
 
     if (alarm_timer->flags & ALARM_FLAG_EXPIRED) {
+        char byte;
+        do {
+            ret = read(alarm_timer_rfd, &byte, sizeof(byte));
+        } while (ret != -1 || errno != EAGAIN);
+
         alarm_timer->flags &= ~(ALARM_FLAG_EXPIRED);
         qemu_rearm_alarm_timer(alarm_timer);
     }


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