[Qemu-devel] [PATCH 3/3] qemu-timer: add qemu_alarm_timer->timer_modified_lock

[Stefan Hajnoczi]

When a timer's expiration time is changed we may need to rearm the alarm
timer.  Rearming is necessary when the nearest deadline changes.

This patch introduces the qemu_alarm_timer->timer_modified boolean and a
lock to protect it.  It moves rearming the alarm timer from inside
qemu_mod_timer_ns() to qemu_run_all_timers() since we cannot rearm
outside the QEMU global mutex.

The following code is dropped because we always kick the main loop now:

  /* Interrupt execution to force deadline recalculation.  */
  qemu_clock_warp(ts->clock);
  if (use_icount) {
      qemu_notify_event();
  }

cpus.c will invoke qemu_clock_warp() again since the main loop ran.

It is now safe to call qemu_mod_timer_ns() for outside the QEMU global
mutex.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 qemu-timer.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index c773af0..9500d12 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -78,6 +78,10 @@ struct qemu_alarm_timer {
 #endif
     bool expired;
     bool pending;
+
+    /* Was the nearest deadline timer modified (possibly by another thread)? */
+    QemuMutex timer_modified_lock;
+    bool timer_modified;
 };
 
 static struct qemu_alarm_timer *alarm_timer;
@@ -371,14 +375,10 @@ void qemu_mod_timer_ns(QEMUTimer *ts, int64_t expire_time)
 
     /* Rearm if necessary  */
     if (pt == &ts->clock->active_timers) {
-        if (!alarm_timer->pending) {
-            qemu_rearm_alarm_timer(alarm_timer);
-        }
-        /* Interrupt execution to force deadline recalculation.  */
-        qemu_clock_warp(ts->clock);
-        if (use_icount) {
-            qemu_notify_event();
-        }
+        qemu_mutex_lock(&alarm_timer->timer_modified_lock);
+        alarm_timer->timer_modified = true;
+        qemu_mutex_unlock(&alarm_timer->timer_modified_lock);
+        qemu_notify_event();
     }
 }
 
@@ -484,6 +484,8 @@ uint64_t qemu_timer_expire_time_ns(QEMUTimer *ts)
 
 void qemu_run_all_timers(void)
 {
+    bool timer_modified;
+
     alarm_timer->pending = false;
 
     /* vm time timers */
@@ -491,11 +493,20 @@ void qemu_run_all_timers(void)
     qemu_run_timers(rt_clock);
     qemu_run_timers(host_clock);
 
+    /* Check if qemu_mod_timer_ns() has been called */
+    qemu_mutex_lock(&alarm_timer->timer_modified_lock);
+    timer_modified = alarm_timer->timer_modified;
+    alarm_timer->timer_modified = false;
+    qemu_mutex_unlock(&alarm_timer->timer_modified_lock);
+
     /* rearm timer, if not periodic */
     if (alarm_timer->expired) {
         alarm_timer->expired = false;
         qemu_rearm_alarm_timer(alarm_timer);
+    } else if (timer_modified) {
+        qemu_rearm_alarm_timer(alarm_timer);
     }
+
 }
 
 #ifdef _WIN32
@@ -805,6 +816,8 @@ int init_timer_alarm(void)
         goto fail;
     }
 
+    qemu_mutex_init(&t->timer_modified_lock);
+
     atexit(quit_timers);
 #ifdef CONFIG_POSIX
     pthread_atfork(NULL, NULL, reinit_timers);
-- 
1.8.1.4