+// SPDX-FileCopyrightText: 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+// SPDX-FileCopyrightText: 2009 Paul E. McKenney, IBM Corporation.
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
#ifndef _URCU_POINTER_STATIC_H
#define _URCU_POINTER_STATIC_H
/*
- * urcu/static/pointer.h
- *
* Userspace RCU header. Operations on pointers.
*
* TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
* RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
*
- * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
- * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
- *
- * This 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.
- *
- * This 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 this library; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
- *
* IBM's contributions to this file may be relicensed under LGPLv2 or later.
*/
* Inserts memory barriers on architectures that require them (currently only
* Alpha) and documents which pointers are protected by RCU.
*
- * The compiler memory barrier in CMM_LOAD_SHARED() ensures that value-speculative
- * optimizations (e.g. VSS: Value Speculation Scheduling) does not perform the
- * data read before the pointer read by speculating the value of the pointer.
- * Correct ordering is ensured because the pointer is read as a volatile access.
- * This acts as a global side-effect operation, which forbids reordering of
- * dependent memory operations. Note that such concern about dependency-breaking
- * optimizations will eventually be taken care of by the "memory_order_consume"
- * addition to forthcoming C++ standard.
+ * With C standards prior to C11/C++11, the compiler memory barrier in
+ * CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
+ * VSS: Value Speculation Scheduling) does not perform the data read
+ * before the pointer read by speculating the value of the pointer.
+ * Correct ordering is ensured because the pointer is read as a volatile
+ * access. This acts as a global side-effect operation, which forbids
+ * reordering of dependent memory operations.
+ *
+ * With C standards C11/C++11, concerns about dependency-breaking
+ * optimizations are taken care of by the "memory_order_consume" atomic
+ * load.
+ *
+ * Use the gcc __atomic_load() rather than C11/C++11 atomic load
+ * explicit because the pointer used as input argument is a pointer,
+ * not an _Atomic type as required by C11/C++11.
+ *
+ * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
+ * volatile access to implement rcu_dereference rather than
+ * memory_order_consume load from the C11/C++11 standards.
+ *
+ * This may improve performance on weakly-ordered architectures where
+ * the compiler implements memory_order_consume as a
+ * memory_order_acquire, which is stricter than required by the
+ * standard.
+ *
+ * Note that using volatile accesses for rcu_dereference may cause
+ * LTO to generate incorrectly ordered code starting from C11/C++11.
*
* Should match rcu_assign_pointer() or rcu_xchg_pointer().
*
* meets the 10-line criterion in LGPL, allowing this function to be
* expanded directly in non-LGPL code.
*/
-#define _rcu_dereference(p) \
- __extension__ \
- ({ \
- __typeof__(p) _________p1 = CMM_LOAD_SHARED(p); \
- cmm_smp_read_barrier_depends(); \
- (_________p1); \
- })
+#if !defined (URCU_DEREFERENCE_USE_VOLATILE) && \
+ ((defined (__cplusplus) && __cplusplus >= 201103L) || \
+ (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
+# define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
+#endif
+
+/*
+ * If p is const (the pointer itself, not what it points to), using
+ * __typeof__(p) would declare a const variable, leading to
+ * -Wincompatible-pointer-types errors. Using the statement expression
+ * makes it an rvalue and gets rid of the const-ness.
+ */
+# define _rcu_dereference(p) \
+ uatomic_load(&(p), CMM_CONSUME)
/**
* _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
* is as expected by "old". If succeeds, returns the previous pointer to the
* using synchronize_rcu(). If fails (unexpected value), returns old (which
* should not be freed !).
*
- * uatomic_cmpxchg() acts as both release and acquire barriers.
+ * uatomic_cmpxchg() acts as both release and acquire barriers on success.
*
* This macro is less than 10 lines long. The intent is that this macro
* meets the 10-line criterion in LGPL, allowing this function to be
({ \
__typeof__(*p) _________pold = (old); \
__typeof__(*p) _________pnew = (_new); \
- uatomic_cmpxchg(p, _________pold, _________pnew); \
- })
+ uatomic_cmpxchg_mo(p, _________pold, _________pnew, \
+ CMM_SEQ_CST, CMM_RELAXED); \
+ });
/**
* _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
__extension__ \
({ \
__typeof__(*p) _________pv = (v); \
- uatomic_xchg(p, _________pv); \
+ uatomic_xchg_mo(p, _________pv, \
+ CMM_SEQ_CST); \
})
-#define _rcu_set_pointer(p, v) \
- do { \
- __typeof__(*p) _________pv = (v); \
- if (!__builtin_constant_p(v) || \
- ((v) != NULL)) \
- cmm_wmb(); \
- uatomic_set(p, _________pv); \
+#define _rcu_set_pointer(p, v) \
+ do { \
+ __typeof__(*p) _________pv = (v); \
+ uatomic_store(p, _________pv, \
+ __builtin_constant_p(v) && (v) == NULL ? \
+ CMM_RELAXED : CMM_RELEASE); \
} while (0)
/**
projects / urcu.git / blobdiff