libevent: fix race in the common case.

third_party/libevent/event.c

Index: third_party/libevent/event.c
diff --git a/third_party/libevent/event.c b/third_party/libevent/event.c
index 125335207d518eb63b6ef0e60453221041d90b92..81d706fe6444a3b1cc6ef63e15ffc2f9b049c816 100644
--- a/third_party/libevent/event.c
+++ b/third_party/libevent/event.c
@@ -107,7 +107,7 @@ static const struct eventop *eventops[] = {
 /* Global state */
 struct event_base *current_base = NULL;
 extern struct event_base *evsignal_base;
-static int use_monotonic;
+static int no_monotonic_clock = 0;

[jln (very slow on Chromium), 2014/04/09 20:13:09]

Isn't it more clear to keep this as "use_monotonic" and statically initialize it
to 1?

"no__monotonic_clock == 0" is harder to read, as a double negation.

[agl, 2014/04/09 20:25:20]

Yea, fair point. I was just aiming for it to be BSS, but you're right that it's
not worth the extra thought when reading the code.

Done.

 
 /* Prototypes */
 static void	event_queue_insert(struct event_base *, struct event *, int);
@@ -120,17 +120,6 @@ static int	timeout_next(struct event_base *, struct timeval **);
 static void	timeout_process(struct event_base *);
 static void	timeout_correct(struct event_base *, struct timeval *);
 
-static void
-detect_monotonic(void)
-{
-#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
-	struct timespec	ts;
-
-	if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0)
-		use_monotonic = 1;
-#endif
-}
-
 static int
 gettime(struct event_base *base, struct timeval *tp)
 {
@@ -140,18 +129,18 @@ gettime(struct event_base *base, struct timeval *tp)
 	}
 
 #if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
-	if (use_monotonic) {
-		struct timespec	ts;
-
-		if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
-			return (-1);
+	struct timespec	ts;
 
+	if (no_monotonic_clock == 0 &&
+	    clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
 		tp->tv_sec = ts.tv_sec;
 		tp->tv_usec = ts.tv_nsec / 1000;
 		return (0);
 	}
 #endif
 
+	no_monotonic_clock = 1;
+
 	return (evutil_gettimeofday(tp, NULL));
 }
 
@@ -175,7 +164,6 @@ event_base_new(void)
 	if ((base = calloc(1, sizeof(struct event_base))) == NULL)
 		event_err(1, "%s: calloc", __func__);
 
-	detect_monotonic();
 	gettime(base, &base->event_tv);
 	
 	min_heap_ctor(&base->timeheap);
@@ -868,7 +856,7 @@ timeout_correct(struct event_base *base, struct timeval *tv)
 	unsigned int size;
 	struct timeval off;
 
-	if (use_monotonic)
+	if (no_monotonic_clock == 0)
 		return;
 
 	/* Check if time is running backwards */