timer: make qemu_clock_enable sync between disable and timer's cb

After disabling the QemuClock, we should make sure that no QemuTimers
are still in flight. To implement that with light overhead, we resort
to QemuEvent. The caller of disabling will wait on QemuEvent of each
timerlist.

Note, qemu_clock_enable(foo,false) can _not_ be called from timer's cb.
Also, the callers of qemu_clock_enable() should be protected by the BQL.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/qemu/timer.h b/include/qemu/timer.h
index 016e29ae95b73401456bd0f6037ce648d4272d68..1254ef729e3f694139613fa59957ce415913f254 100644 (file)
--- a/include/qemu/timer.h
+++ b/include/qemu/timer.h
@@ -189,6 +189,12 @@ void qemu_clock_notify(QEMUClockType type);
  * @enabled: true to enable, false to disable
  *
  * Enable or disable a clock
+ * Disabling the clock will wait for related timerlists to stop
+ * executing qemu_run_timers.  Thus, this functions should not
+ * be used from the callback of a timer that is based on @clock.
+ * Doing so would cause a deadlock.
+ *
+ * Caller should hold BQL.
  */
 void qemu_clock_enable(QEMUClockType type, bool enabled);
 

diff --git a/qemu-timer.c b/qemu-timer.c
index 6b62e88669e54f2bbcb969fee2a3445fabb0ff7b..2b533da2c74e67f659429d2ae4a847aff6d9aa43 100644 (file)
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -45,6 +45,7 @@
 /* timers */
 
 typedef struct QEMUClock {
+    /* We rely on BQL to protect the timerlists */
     QLIST_HEAD(, QEMUTimerList) timerlists;
 
     NotifierList reset_notifiers;
@@ -71,6 +72,9 @@ struct QEMUTimerList {
     QLIST_ENTRY(QEMUTimerList) list;
     QEMUTimerListNotifyCB *notify_cb;
     void *notify_opaque;
+
+    /* lightweight method to mark the end of timerlist's running */
+    QemuEvent timers_done_ev;
 };
 
 /**
@@ -99,6 +103,7 @@ QEMUTimerList *timerlist_new(QEMUClockType type,
     QEMUClock *clock = qemu_clock_ptr(type);
 
     timer_list = g_malloc0(sizeof(QEMUTimerList));
+    qemu_event_init(&timer_list->timers_done_ev, false);
     timer_list->clock = clock;
     timer_list->notify_cb = cb;
     timer_list->notify_opaque = opaque;
@@ -143,13 +148,25 @@ void qemu_clock_notify(QEMUClockType type)
     }
 }
 
+/* Disabling the clock will wait for related timerlists to stop
+ * executing qemu_run_timers.  Thus, this functions should not
+ * be used from the callback of a timer that is based on @clock.
+ * Doing so would cause a deadlock.
+ *
+ * Caller should hold BQL.
+ */
 void qemu_clock_enable(QEMUClockType type, bool enabled)
 {
     QEMUClock *clock = qemu_clock_ptr(type);
+    QEMUTimerList *tl;
     bool old = clock->enabled;
     clock->enabled = enabled;
     if (enabled && !old) {
         qemu_clock_notify(type);
+    } else if (!enabled && old) {
+        QLIST_FOREACH(tl, &clock->timerlists, list) {
+            qemu_event_wait(&tl->timers_done_ev);
+        }
     }
 }
 
@@ -403,8 +420,9 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
     QEMUTimerCB *cb;
     void *opaque;
 
+    qemu_event_reset(&timer_list->timers_done_ev);
     if (!timer_list->clock->enabled) {
-        return progress;
+        goto out;
     }
 
     current_time = qemu_clock_get_ns(timer_list->clock->type);
@@ -428,6 +446,9 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         cb(opaque);
         progress = true;
     }
+
+out:
+    qemu_event_set(&timer_list->timers_done_ev);
     return progress;
 }