X-Git-Url: https://git.lttng.org/?p=lttng-tools.git;a=blobdiff_plain;f=src%2Fcommon%2Fwaiter.cpp;fp=src%2Fcommon%2Fwaiter.cpp;h=aca88eb4db75ade4fa28b971d4616be68bd92742;hp=0000000000000000000000000000000000000000;hb=a6bc4ca9d659caf016ef932fcd944029737ac57c;hpb=97535efaa975ca52bf02c2d5e76351bfd2e3defa

diff --git a/src/common/waiter.cpp b/src/common/waiter.cpp
new file mode 100644
index 000000000..aca88eb4d
--- /dev/null
+++ b/src/common/waiter.cpp
@@ -0,0 +1,108 @@
+/*
+ * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ *
+ * SPDX-License-Identifier: LGPL-2.1-only
+ *
+ */
+
+#include "waiter.h"
+#include <urcu/uatomic.h>
+#include <urcu/futex.h>
+#include "error.h"
+#include <poll.h>
+
+/*
+ * Number of busy-loop attempts before waiting on futex.
+ */
+#define WAIT_ATTEMPTS 1000
+
+enum waiter_state {
+	/* WAITER_WAITING is compared directly (futex compares it). */
+	WAITER_WAITING =	0,
+	/* non-zero are used as masks. */
+	WAITER_WOKEN_UP =	(1 << 0),
+	WAITER_RUNNING =	(1 << 1),
+	WAITER_TEARDOWN =	(1 << 2),
+};
+
+void lttng_waiter_init(struct lttng_waiter *waiter)
+{
+	cds_wfs_node_init(&waiter->wait_queue_node);
+	uatomic_set(&waiter->state, WAITER_WAITING);
+	cmm_smp_mb();
+}
+
+/*
+ * User must init "waiter" before passing its memory to waker thread.
+ */
+void lttng_waiter_wait(struct lttng_waiter *waiter)
+{
+	unsigned int i;
+
+	DBG("Beginning of waiter wait period");
+	/* Load and test condition before read state */
+	cmm_smp_rmb();
+	for (i = 0; i < WAIT_ATTEMPTS; i++) {
+		if (uatomic_read(&waiter->state) != WAITER_WAITING) {
+			goto skip_futex_wait;
+		}
+		caa_cpu_relax();
+	}
+	while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
+			NULL, NULL, 0)) {
+		switch (errno) {
+		case EWOULDBLOCK:
+			/* Value already changed. */
+			goto skip_futex_wait;
+		case EINTR:
+			/* Retry if interrupted by signal. */
+			break;	/* Get out of switch. */
+		default:
+			/* Unexpected error. */
+			PERROR("futex_noasync");
+			abort();
+		}
+	}
+skip_futex_wait:
+
+	/* Tell waker thread than we are running. */
+	uatomic_or(&waiter->state, WAITER_RUNNING);
+
+	/*
+	 * Wait until waker thread lets us know it's ok to tear down
+	 * memory allocated for struct lttng_waiter.
+	 */
+	for (i = 0; i < WAIT_ATTEMPTS; i++) {
+		if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
+			break;
+		}
+		caa_cpu_relax();
+	}
+	while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
+		poll(NULL, 0, 10);
+	}
+	LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
+	DBG("End of waiter wait period");
+}
+
+/*
+ * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
+ * execution. In this scheme, the waiter owns the node memory, and we only allow
+ * it to free this memory when it sees the WAITER_TEARDOWN flag.
+ */
+void lttng_waiter_wake_up(struct lttng_waiter *waiter)
+{
+	cmm_smp_mb();
+	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) {
+			PERROR("futex_noasync");
+			abort();
+		}
+	}
+	/* Allow teardown of struct urcu_wait memory. */
+	uatomic_or(&waiter->state, WAITER_TEARDOWN);
+}