X-Git-Url: https://git.lttng.org/?p=lttng-tools.git;a=blobdiff_plain;f=src%2Fcommon%2Ffutex.c;fp=src%2Fcommon%2Ffutex.c;h=f1b33ea34dabc3c5cbe8352d98280a5ba9c553c7;hp=384e957a22d0406751c13dd03f02edd22bb0b768;hb=dce89628fd85a875a8dc511d861057f218f3c1c8;hpb=d53ea4e443912f7b3fe89da38e3374db2a1a3bf8

diff --git a/src/common/futex.c b/src/common/futex.c
index 384e957a2..f1b33ea34 100644
--- a/src/common/futex.c
+++ b/src/common/futex.c
@@ -30,18 +30,22 @@
  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  * "nto1" added to all function signature.
  *
- * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
- * git tree for a detail example of this scheme being used. futex_async() is
- * the urcu wrapper over the futex() sycall.
- *
- * There is also a formal verification available in the git tree.
- *
- *   branch: formal-model
- *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
- *
- * Ref: git://git.lttng.org/userspace-rcu.git
+ * The code is adapted from the adaptative busy wait/wake_up scheme used in
+ * liburcu.
  */
 
+/* Number of busy-loop attempts before waiting on futex. */
+#define FUTEX_WAIT_ATTEMPTS 1000
+
+enum futex_wait_state {
+	/* FUTEX_WAIT_WAITING is compared directly (futex() compares it). */
+	FUTEX_WAIT_WAITING =	0,
+	/* non-zero are used as masks. */
+	FUTEX_WAIT_WAKEUP =	(1 << 0),
+	FUTEX_WAIT_RUNNING =	(1 << 1),
+	FUTEX_WAIT_TEARDOWN =	(1 << 2),
+};
+
 /*
  * Update futex according to active or not. This scheme is used to wake every
  * libust waiting on the shared memory map futex hence the INT_MAX used in the
@@ -71,7 +75,7 @@ void futex_wait_update(int32_t *futex, int active)
 LTTNG_HIDDEN
 void futex_nto1_prepare(int32_t *futex)
 {
-	uatomic_set(futex, -1);
+	uatomic_set(futex, FUTEX_WAIT_WAITING);
 	cmm_smp_mb();
 
 	DBG("Futex n to 1 prepare done");
@@ -83,25 +87,47 @@ void futex_nto1_prepare(int32_t *futex)
 LTTNG_HIDDEN
 void futex_nto1_wait(int32_t *futex)
 {
-	cmm_smp_mb();
+	unsigned int i;
 
-	if (uatomic_read(futex) != -1)
-		goto end;
-	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
+	/* Load and test condition before read state */
+	cmm_smp_rmb();
+	for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) {
+		if (uatomic_read(futex) != FUTEX_WAIT_WAITING)
+			goto skip_futex_wait;
+		caa_cpu_relax();
+	}
+	while (futex_noasync(futex, FUTEX_WAIT, FUTEX_WAIT_WAITING,
+			NULL, NULL, 0)) {
 		switch (errno) {
 		case EWOULDBLOCK:
 			/* Value already changed. */
-			goto end;
+			goto skip_futex_wait;
 		case EINTR:
 			/* Retry if interrupted by signal. */
 			break;	/* Get out of switch. */
 		default:
 			/* Unexpected error. */
-			PERROR("futex_async");
+			PERROR("futex");
 			abort();
 		}
 	}
-end:
+skip_futex_wait:
+
+	/* Tell waker thread than we are running. */
+	uatomic_or(futex, FUTEX_WAIT_RUNNING);
+
+	/*
+	 * Wait until waker thread lets us know it's ok to tear down
+	 * memory allocated for the futex.
+	 */
+	for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) {
+		if (uatomic_read(futex) & FUTEX_WAIT_TEARDOWN)
+			break;
+		caa_cpu_relax();
+	}
+	while (!(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN))
+		poll(NULL, 0, 10);
+	assert(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN);
 	DBG("Futex n to 1 wait done");
 }
 
@@ -111,13 +137,16 @@ end:
 LTTNG_HIDDEN
 void futex_nto1_wake(int32_t *futex)
 {
-	if (caa_unlikely(uatomic_read(futex) != -1))
-		goto end;
-	uatomic_set(futex, 0);
-	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
-		PERROR("futex_async");
-		abort();
+	cmm_smp_mb();
+	uatomic_set(futex, FUTEX_WAIT_WAKEUP);
+	if (!(uatomic_read(futex) & FUTEX_WAIT_RUNNING)) {
+		if (futex_noasync(futex, FUTEX_WAKE, 1,
+				NULL, NULL, 0) < 0) {
+			PERROR("futex_noasync");
+			abort();
+		}
 	}
-end:
+	/* Allow teardown of futex. */
+	uatomic_or(futex, FUTEX_WAIT_TEARDOWN);
 	DBG("Futex n to 1 wake done");
 }