getenv: make getenv helper init state mt-safe

Change-Id: Idebb709fe4dd5501dfe657ea23fc6a67a65ce1ff
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/src/common/getenv.c b/src/common/getenv.c
index 23029508b161d126590d8ed3bbcc943e0e23aa8c..bc7a4d4ac98ce2f7ccc434e94727d0778ce7338a 100644 (file)
--- a/src/common/getenv.c
+++ b/src/common/getenv.c
@@ -9,6 +9,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <sys/types.h>
+#include <pthread.h>
 #include <urcu/system.h>
 #include "common/logging.h"
 #include "common/macros.h"
@@ -25,8 +26,9 @@ struct lttng_env {
        char *value;
 };
 
-static
-int lttng_ust_getenv_is_init = 0;
+/* lttng_ust_getenv_init_mutex provides mutual exclusion for initialization. */
+static pthread_mutex_t lttng_ust_getenv_init_mutex = PTHREAD_MUTEX_INITIALIZER;
+static int lttng_ust_getenv_is_init = 0;
 
 static struct lttng_env lttng_env[] = {
        /*
@@ -88,8 +90,25 @@ void lttng_ust_getenv_init(void)
 {
        size_t i;
 
-       if (CMM_LOAD_SHARED(lttng_ust_getenv_is_init))
+       /*
+        * Return early if the init has already completed.
+        */
+       if (CMM_LOAD_SHARED(lttng_ust_getenv_is_init)) {
+               /*
+                * Load lttng_ust_getenv_is_init before reading environment cache.
+                */
+               cmm_smp_rmb();
                return;
+       }
+
+       pthread_mutex_lock(&lttng_ust_getenv_init_mutex);
+
+       /*
+        * Check again if the init has completed in another thread now that we
+        * have acquired the mutex.
+        */
+       if (lttng_ust_getenv_is_init)
+               goto end_init;
 
        for (i = 0; i < LTTNG_ARRAY_SIZE(lttng_env); i++) {
                struct lttng_env *e = &lttng_env[i];
@@ -101,5 +120,12 @@ void lttng_ust_getenv_init(void)
                }
                e->value = getenv(e->key);
        }
+
+       /*
+        * Store environment cache before setting lttng_ust_getenv_is_init to 1.
+        */
+       cmm_smp_wmb();
        CMM_STORE_SHARED(lttng_ust_getenv_is_init, 1);
+end_init:
+       pthread_mutex_unlock(&lttng_ust_getenv_init_mutex);
 }