common: compile libcommon as C++

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

diff --git a/src/common/futex.cpp b/src/common/futex.cpp
new file mode 100644 (file)
index 0000000..d0f95ab
--- /dev/null
+++ b/src/common/futex.cpp
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ *
+ */
+
+#define _LGPL_SOURCE
+#include <limits.h>
+#include <unistd.h>
+#include <urcu.h>
+#include <urcu/futex.h>
+
+#include <common/common.h>
+
+#include "futex.h"
+
+/*
+ * 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
+ */
+
+/*
+ * 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
+ * futex() call. If active, we set the value and wake everyone else we indicate
+ * that we are gone (cleanup() case).
+ */
+void futex_wait_update(int32_t *futex, int active)
+{
+       if (active) {
+               uatomic_set(futex, 1);
+               if (futex_async(futex, FUTEX_WAKE,
+                               INT_MAX, NULL, NULL, 0) < 0) {
+                       PERROR("futex_async");
+                       abort();
+               }
+       } else {
+               uatomic_set(futex, 0);
+       }
+
+       DBG("Futex wait update active %d", active);
+}
+
+/*
+ * Prepare futex.
+ */
+void futex_nto1_prepare(int32_t *futex)
+{
+       uatomic_set(futex, -1);
+       cmm_smp_mb();
+
+       DBG("Futex n to 1 prepare done");
+}
+
+/*
+ * Wait futex.
+ */
+void futex_nto1_wait(int32_t *futex)
+{
+       cmm_smp_mb();
+
+       if (uatomic_read(futex) != -1)
+               goto end;
+       while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
+               switch (errno) {
+               case EWOULDBLOCK:
+                       /* Value already changed. */
+                       goto end;
+               case EINTR:
+                       /* Retry if interrupted by signal. */
+                       break;  /* Get out of switch. */
+               default:
+                       /* Unexpected error. */
+                       PERROR("futex_async");
+                       abort();
+               }
+       }
+end:
+       DBG("Futex n to 1 wait done");
+}
+
+/*
+ * Wake 1 futex.
+ */
+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();
+       }
+end:
+       DBG("Futex n to 1 wake done");
+}