X-Git-Url: https://git.lttng.org/?a=blobdiff_plain;f=src%2Fcommon%2Fwaiter.cpp;h=c0ea3392fceea52f1438b6e839c1742e7348503b;hb=00c2aa6f664e9ec249fd297fb01309477917d029;hp=3ddb68feb7c76dd91f09bec9d7664c02224627a2;hpb=c9e313bc594f40a86eed237dce222c0fc99c957f;p=lttng-tools.git

diff --git a/src/common/waiter.cpp b/src/common/waiter.cpp
index 3ddb68feb..c0ea3392f 100644
--- 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();
 		}