[Qemu-devel] [RFC PATCH 3/3] qemu-timer: do not take the lock in timer_pending

[Paolo Bonzini <pbonzini@redhat.com>]

We can deduce the result from expire_time, by making it always -1 if
the timer is not in the active_timers list.  We need to check against
negative times passed to timer_mod_ns; clamping them to zero is not
a problem because the only clock that has a zero value at VM startup
is QEMU_CLOCK_VIRTUAL, and it is monotonic so it cannot be non-zero.
QEMU_CLOCK_HOST, instead, is not monotonic but it cannot go to negative
values unless the host time is seriously screwed up and points to
the 1960s.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 qemu-timer.c | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index aa22801..807319a 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -312,6 +312,7 @@ void timer_init(QEMUTimer *ts,
     ts->cb = cb;
     ts->opaque = opaque;
     ts->scale = scale;
+    ts->expire_time = -1;
 }
 
 void timer_free(QEMUTimer *ts)
@@ -324,6 +325,7 @@ static void timer_del_locked(QEMUTimerList *timer_list, QEMUTimer *ts)
 {
     QEMUTimer **pt, *t;
 
+    ts->expire_time = -1;
     pt = &timer_list->active_timers;
     for(;;) {
         t = *pt;
@@ -366,7 +368,7 @@ void timer_mod_ns(QEMUTimer *ts, int64_t expire_time)
         }
         pt = &t->next;
     }
-    ts->expire_time = expire_time;
+    ts->expire_time = MAX(expire_time, 0);
     ts->next = *pt;
     *pt = ts;
     qemu_mutex_unlock(&timer_list->active_timers_lock);
@@ -386,19 +388,7 @@ void timer_mod(QEMUTimer *ts, int64_t expire_time)
 
 bool timer_pending(QEMUTimer *ts)
 {
-    QEMUTimerList *timer_list = ts->timer_list;
-    QEMUTimer *t;
-    bool found = false;
-
-    qemu_mutex_lock(&timer_list->active_timers_lock);
-    for (t = timer_list->active_timers; t != NULL; t = t->next) {
-        if (t == ts) {
-            found = true;
-            break;
-        }
-    }
-    qemu_mutex_unlock(&timer_list->active_timers_lock);
-    return found;
+    return ts->expire_time >= 0;
 }
 
 bool timer_expired(QEMUTimer *timer_head, int64_t current_time)
@@ -430,6 +420,7 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         /* remove timer from the list before calling the callback */
         timer_list->active_timers = ts->next;
         ts->next = NULL;
+        ts->expire_time = -1;
         cb = ts->cb;
         opaque = ts->opaque;
         qemu_mutex_unlock(&timer_list->active_timers_lock);
-- 
1.8.3.1