[Qemu-devel] [PATCH v5 4/4] timer: make qemu_clock_enable sync between disable and timer's cb

[Liu Ping Fan <qemulist@gmail.com>]

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.
And the callers of qemu_clock_enable() should be sync by themselves,
not protected by this patch.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
 include/qemu/timer.h |  6 ++++++
 qemu-timer.c         | 21 ++++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/include/qemu/timer.h b/include/qemu/timer.h
index bb1de23..c7fa83a 100644
--- a/include/qemu/timer.h
+++ b/include/qemu/timer.h
@@ -185,6 +185,12 @@ 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 95ff47f..3a828aa 100644
--- 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;
@@ -70,6 +71,8 @@ struct QEMUTimerList {
     QLIST_ENTRY(QEMUTimerList) list;
     QEMUTimerListNotifyCB *notify_cb;
     void *notify_opaque;
+    /* light weight method to mark the end of timerlist's running */
+    QemuEvent timers_done_ev;
 };
 
 /**
@@ -98,6 +101,7 @@ 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;
@@ -140,13 +144,25 @@ 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);
+        }
     }
 }
 
@@ -373,8 +389,10 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
     QEMUTimer *ts;
     int64_t current_time;
     bool progress = false;
-   
+
+    qemu_event_reset(&timer_list->timers_done_ev);
     if (!timer_list->clock->enabled) {
+        qemu_event_set(&timer_list->timers_done_ev);
         return progress;
     }
 
@@ -392,6 +410,7 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         ts->cb(ts->opaque);
         progress = true;
     }
+    qemu_event_set(&timer_list->timers_done_ev);
     return progress;
 }
 
-- 
1.8.1.4