Build fix: warning: clang warns that uuid_scan may be uninitialized

[lttng-tools.git] / src / common / waiter.cpp

diff --git a/src/common/waiter.cpp b/src/common/waiter.cpp
index 3ddb68feb7c76dd91f09bec9d7664c02224627a2..c0ea3392fceea52f1438b6e839c1742e7348503b 100644 (file)
--- a/src/common/waiter.cpp
+++ b/src/common/waiter.cpp
@@ -6,11 +6,12 @@
  *
  */
 
-#include "waiter.hpp"
-#include <urcu/uatomic.h>
-#include <urcu/futex.h>
 #include "error.hpp"
+#include "waiter.hpp"
+
 #include <poll.h>
+#include <urcu/futex.h>
+#include <urcu/uatomic.h>
 
 /*
  * Number of busy-loop attempts before waiting on futex.
@@ -19,11 +20,11 @@
 
 enum waiter_state {
        /* WAITER_WAITING is compared directly (futex compares it). */
-       WAITER_WAITING =        0,
+       WAITER_WAITING = 0,
        /* non-zero are used as masks. */
-       WAITER_WOKEN_UP =       (1 << 0),
-       WAITER_RUNNING =        (1 << 1),
-       WAITER_TEARDOWN =       (1 << 2),
+       WAITER_WOKEN_UP = (1 << 0),
+       WAITER_RUNNING = (1 << 1),
+       WAITER_TEARDOWN = (1 << 2),
 };
 
 void lttng_waiter_init(struct lttng_waiter *waiter)
@@ -49,15 +50,26 @@ void lttng_waiter_wait(struct lttng_waiter *waiter)
                }
                caa_cpu_relax();
        }
-       while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
-                       NULL, NULL, 0)) {
+       while (uatomic_read(&waiter->state) == WAITER_WAITING) {
+               if (!futex_noasync(
+                           &waiter->state, FUTEX_WAIT, WAITER_WAITING, nullptr, nullptr, 0)) {
+                       /*
+                        * Prior queued wakeups queued by unrelated code
+                        * using the same address can cause futex wait to
+                        * return 0 even through the futex value is still
+                        * WAITER_WAITING (spurious wakeups). Check
+                        * the value again in user-space to validate
+                        * whether it really differs from WAITER_WAITING.
+                        */
+                       continue;
+               }
                switch (errno) {
-               case EWOULDBLOCK:
+               case EAGAIN:
                        /* Value already changed. */
                        goto skip_futex_wait;
                case EINTR:
                        /* Retry if interrupted by signal. */
-                       break;  /* Get out of switch. */
+                       break; /* Get out of switch. Check again. */
                default:
                        /* Unexpected error. */
                        PERROR("futex_noasync");
@@ -80,7 +92,7 @@ skip_futex_wait:
                caa_cpu_relax();
        }
        while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
-               poll(NULL, 0, 10);
+               poll(nullptr, 0, 10);
        }
        LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
        DBG("End of waiter wait period");
@@ -97,8 +109,7 @@ void lttng_waiter_wake_up(struct lttng_waiter *waiter)
        LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
        uatomic_set(&waiter->state, WAITER_WOKEN_UP);
        if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
-               if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
-                               NULL, NULL, 0) < 0) {
+               if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
                        PERROR("futex_noasync");
                        abort();
                }