Bugfix: correctly handle spurious wakeups in pthread_cond_timedwait

This fixes problems storing data in rrd files on NetBSD 5.0 with more
than one CPU.

Signed-off-by: Florian Forster <octo@huhu.verplant.org>

diff --git a/src/plugin.c b/src/plugin.c
index 11a0ef6..c163ca7 100644 (file)
--- a/src/plugin.c
+++ b/src/plugin.c
@@ -302,6 +302,13 @@ static int plugin_load_file (char *file)
        return (0);
 }
 
+static _Bool timeout_reached(struct timespec timeout)
+{
+       struct timeval now;
+       gettimeofday(&now, NULL);
+       return (now.tv_sec >= timeout.tv_sec && now.tv_usec >= (timeout.tv_nsec / 1000));
+}
+
 static void *plugin_read_thread (void __attribute__((unused)) *args)
 {
        while (read_loop != 0)
@@ -310,6 +317,7 @@ static void *plugin_read_thread (void __attribute__((unused)) *args)
                struct timeval now;
                int status;
                int rf_type;
+               int rc;
 
                /* Get the read function that needs to be read next. */
                rf = c_head_get_root (read_heap);
@@ -345,8 +353,16 @@ static void *plugin_read_thread (void __attribute__((unused)) *args)
                /* sleep until this entry is due,
                 * using pthread_cond_timedwait */
                pthread_mutex_lock (&read_lock);
-               pthread_cond_timedwait (&read_cond, &read_lock,
+               /* In pthread_cond_timedwait, spurious wakeups are possible
+                * (and really happen, at least on NetBSD with > 1 CPU), thus
+                * we need to re-evaluate the condition every time
+                * pthread_cond_timedwait returns. */
+               rc = 0;
+               while (!timeout_reached(rf->rf_next_read) && rc == 0) {
+                       rc = pthread_cond_timedwait (&read_cond, &read_lock,
                                &rf->rf_next_read);
+               }
+
                /* Must hold `real_lock' when accessing `rf->rf_type'. */
                rf_type = rf->rf_type;
                pthread_mutex_unlock (&read_lock);