Implement allocate_once for atomic initialization with allocation

1 files changed, 59 insertions, 0 deletions

diff --git a/misc/allocate_once.c b/misc/allocate_once.c
new file mode 100644
index 0000000000..2108014604
--- /dev/null
+++ b/misc/allocate_once.c
@@ -0,0 +1,59 @@
+/* Concurrent allocation and initialization of a pointer.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <allocate_once.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+void *
+__libc_allocate_once_slow (void **place, void *(*allocate) (void *closure),
+                           void (*deallocate) (void *closure, void *ptr),
+                           void *closure)
+{
+  void *result = allocate (closure);
+  if (result == NULL)
+    return NULL;
+
+  /* This loop implements a strong CAS on *place, with acquire-release
+     MO semantics, from a weak CAS with relaxed-release MO.  */
+  while (true)
+    {
+      /* Synchronizes with the acquire MO load in allocate_once.  */
+      void *expected = NULL;
+      if (atomic_compare_exchange_weak_release (place, &expected, result))
+        return result;
+
+      /* The failed CAS has relaxed MO semantics, so perform another
+         acquire MO load.  */
+      void *other_result = atomic_load_acquire (place);
+      if (other_result == NULL)
+        /* Spurious failure.  Try again.  */
+        continue;
+
+      /* We lost the race.  Free what we allocated and return the
+         other result.  */
+      if (deallocate == NULL)
+        free (result);
+      else
+        deallocate (closure, result);
+      return other_result;
+    }
+
+  return result;
+}
+libc_hidden_def (__libc_allocate_once_slow)