From 5f7b841d3aebdccc2baed27cb4b22ddb08cd7c0c Mon Sep 17 00:00:00 2001 From: Florian Weimer Date: Wed, 23 May 2018 14:16:18 +0200 Subject: Implement allocate_once for atomic initialization with allocation --- misc/allocate_once.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 59 insertions(+) create mode 100644 misc/allocate_once.c (limited to 'misc/allocate_once.c') 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 + . */ + +#include +#include +#include + +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) -- cgit 1.4.1