[urcu.git] / include / urcu / static / pointer.h

// 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

/*
 * 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.
 *
 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
 */

#include <urcu/compiler.h>
#include <urcu/arch.h>
#include <urcu/system.h>
#include <urcu/uatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

/**
 * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
 * into a RCU read-side critical section. The pointer can later be safely
 * dereferenced within the critical section.
 *
 * This ensures that the pointer copy is invariant thorough the whole critical
 * section.
 *
 * Inserts memory barriers on architectures that require them (currently only
 * Alpha) and documents which pointers are protected by RCU.
 *
 * 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().
 *
 * 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
 * expanded directly in non-LGPL code.
 */

#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.
 */
#ifdef __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
# define _rcu_dereference(p) __extension__ ({						\
				__typeof__(__extension__ ({				\
					__typeof__(p) __attribute__((unused)) _________p0 = { 0 }; \
					_________p0;					\
				})) _________p1;					\
				__atomic_load(&(p), &_________p1, __ATOMIC_CONSUME);	\
				(_________p1);						\
			})
#else
# define _rcu_dereference(p) __extension__ ({						\
				__typeof__(p) _________p1 = CMM_LOAD_SHARED(p);		\
				cmm_smp_read_barrier_depends();				\
				(_________p1);						\
			})
#endif

/**
 * _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
 * data structure, which can be safely freed after waiting for a quiescent state
 * using synchronize_rcu(). If fails (unexpected value), returns old (which
 * should not be freed !).
 *
 * uatomic_cmpxchg() acts as both release and acquire barriers.
 *
 * 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
 * expanded directly in non-LGPL code.
 */
#define _rcu_cmpxchg_pointer(p, old, _new)				\
	__extension__							\
	({								\
		__typeof__(*p) _________pold = (old);			\
		__typeof__(*p) _________pnew = (_new);			\
		uatomic_cmpxchg(p, _________pold, _________pnew);	\
	})

/**
 * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
 * pointer to the data structure, which can be safely freed after waiting for a
 * quiescent state using synchronize_rcu().
 *
 * uatomic_xchg() acts as both release and acquire barriers.
 *
 * 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
 * expanded directly in non-LGPL code.
 */
#define _rcu_xchg_pointer(p, v)				\
	__extension__					\
	({						\
		__typeof__(*p) _________pv = (v);	\
		uatomic_xchg(p, _________pv);		\
	})


#define _rcu_set_pointer(p, v)				\
	do {						\
		__typeof__(*p) _________pv = (v);	\
		if (!__builtin_constant_p(v) || 	\
		    ((v) != NULL))			\
			cmm_wmb();				\
		uatomic_set(p, _________pv);		\
	} while (0)

/**
 * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
 * meant to be read by RCU read-side critical sections. Returns the assigned
 * value.
 *
 * Documents which pointers will be dereferenced by RCU read-side critical
 * sections and adds the required memory barriers on architectures requiring
 * them. It also makes sure the compiler does not reorder code initializing the
 * data structure before its publication.
 *
 * Should match rcu_dereference().
 *
 * 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
 * expanded directly in non-LGPL code.
 */
#define _rcu_assign_pointer(p, v)	_rcu_set_pointer(&(p), v)

#ifdef __cplusplus
}
#endif

#endif /* _URCU_POINTER_STATIC_H */
Commit	Line	Data
d3d3857f MJ	1	// SPDX-FileCopyrightText: 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	2	// SPDX-FileCopyrightText: 2009 Paul E. McKenney, IBM Corporation.
	3	//
	4	// SPDX-License-Identifier: LGPL-2.1-or-later
	5
7e30abe3 MD	6	#ifndef _URCU_POINTER_STATIC_H
	7	#define _URCU_POINTER_STATIC_H
	8
	9	/*
7e30abe3 MD	10	* Userspace RCU header. Operations on pointers.
7e30abe3 MD	11	*
a5a9f428 PM	12	* TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
a5a9f428 PM	13	* RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
7e30abe3	14	*
7e30abe3 MD	15	* IBM's contributions to this file may be relicensed under LGPLv2 or later.
	16	*/
	17
	18	#include <urcu/compiler.h>
	19	#include <urcu/arch.h>
	20	#include <urcu/system.h>
a2e7bf9c	21	#include <urcu/uatomic.h>
7e30abe3	22
36bc70a8 MD	23	#ifdef __cplusplus
36bc70a8 MD	24	extern "C" {
67ecffc0	25	#endif
36bc70a8	26
7e30abe3 MD	27	/**
	28	* _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
	29	* into a RCU read-side critical section. The pointer can later be safely
	30	* dereferenced within the critical section.
	31	*
	32	* This ensures that the pointer copy is invariant thorough the whole critical
	33	* section.
	34	*
	35	* Inserts memory barriers on architectures that require them (currently only
	36	* Alpha) and documents which pointers are protected by RCU.
	37	*
380f4b19 MD	38	* With C standards prior to C11/C++11, the compiler memory barrier in
	39	* CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
	40	* VSS: Value Speculation Scheduling) does not perform the data read
	41	* before the pointer read by speculating the value of the pointer.
	42	* Correct ordering is ensured because the pointer is read as a volatile
	43	* access. This acts as a global side-effect operation, which forbids
	44	* reordering of dependent memory operations.
	45	*
	46	* With C standards C11/C++11, concerns about dependency-breaking
	47	* optimizations are taken care of by the "memory_order_consume" atomic
	48	* load.
	49	*
3abcdbd1 MD	50	* Use the gcc __atomic_load() rather than C11/C++11 atomic load
	51	* explicit because the pointer used as input argument is a pointer,
	52	* not an _Atomic type as required by C11/C++11.
	53	*
380f4b19 MD	54	* By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
	55	* volatile access to implement rcu_dereference rather than
	56	* memory_order_consume load from the C11/C++11 standards.
	57	*
	58	* This may improve performance on weakly-ordered architectures where
	59	* the compiler implements memory_order_consume as a
	60	* memory_order_acquire, which is stricter than required by the
	61	* standard.
	62	*
	63	* Note that using volatile accesses for rcu_dereference may cause
	64	* LTO to generate incorrectly ordered code starting from C11/C++11.
7e30abe3 MD	65	*
7e30abe3 MD	66	* Should match rcu_assign_pointer() or rcu_xchg_pointer().
a5a9f428 PM	67	*
	68	* This macro is less than 10 lines long. The intent is that this macro
	69	* meets the 10-line criterion in LGPL, allowing this function to be
	70	* expanded directly in non-LGPL code.
7e30abe3	71	*/
380f4b19	72
3abcdbd1 MD	73	#if !defined (URCU_DEREFERENCE_USE_VOLATILE) && \
	74	((defined (__cplusplus) && __cplusplus >= 201103L) \|\| \
	75	(defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
	76	# define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
380f4b19 MD	77	#endif
380f4b19 MD	78
1e41ec3b SM	79	/*
	80	* If p is const (the pointer itself, not what it points to), using
	81	* __typeof__(p) would declare a const variable, leading to
2d466a63 SM	82	* -Wincompatible-pointer-types errors. Using the statement expression
2d466a63 SM	83	* makes it an rvalue and gets rid of the const-ness.
1e41ec3b	84	*/
3abcdbd1 MD	85	#ifdef __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
3abcdbd1 MD	86	# define _rcu_dereference(p) __extension__ ({ \
2d466a63	87	__typeof__(__extension__ ({ \
fada682f	88	__typeof__(p) __attribute__((unused)) _________p0 = { 0 }; \
2d466a63 SM	89	_________p0; \
2d466a63 SM	90	})) _________p1; \
3abcdbd1 MD	91	__atomic_load(&(p), &_________p1, __ATOMIC_CONSUME); \
	92	(_________p1); \
	93	})
	94	#else
	95	# define _rcu_dereference(p) __extension__ ({ \
	96	__typeof__(p) _________p1 = CMM_LOAD_SHARED(p); \
	97	cmm_smp_read_barrier_depends(); \
	98	(_________p1); \
	99	})
	100	#endif
7e30abe3 MD	101
	102	/**
	103	* _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
	104	* is as expected by "old". If succeeds, returns the previous pointer to the
	105	* data structure, which can be safely freed after waiting for a quiescent state
	106	* using synchronize_rcu(). If fails (unexpected value), returns old (which
	107	* should not be freed !).
a5a9f428	108	*
2f10494a MD	109	* uatomic_cmpxchg() acts as both release and acquire barriers.
2f10494a MD	110	*
a5a9f428 PM	111	* This macro is less than 10 lines long. The intent is that this macro
	112	* meets the 10-line criterion in LGPL, allowing this function to be
	113	* expanded directly in non-LGPL code.
7e30abe3	114	*/
2b5554c9	115	#define _rcu_cmpxchg_pointer(p, old, _new) \
1b85da85	116	__extension__ \
2b5554c9	117	({ \
bdffa73a MD	118	__typeof__(*p) _________pold = (old); \
bdffa73a MD	119	__typeof__(*p) _________pnew = (_new); \
bf9de1b7	120	uatomic_cmpxchg(p, _________pold, _________pnew); \
7e30abe3 MD	121	})
	122
	123	/**
	124	* _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
	125	* pointer to the data structure, which can be safely freed after waiting for a
	126	* quiescent state using synchronize_rcu().
a5a9f428	127	*
2f10494a MD	128	* uatomic_xchg() acts as both release and acquire barriers.
2f10494a MD	129	*
a5a9f428 PM	130	* This macro is less than 10 lines long. The intent is that this macro
	131	* meets the 10-line criterion in LGPL, allowing this function to be
	132	* expanded directly in non-LGPL code.
7e30abe3	133	*/
7e30abe3	134	#define _rcu_xchg_pointer(p, v) \
1b85da85	135	__extension__ \
7e30abe3	136	({ \
bdffa73a	137	__typeof__(*p) _________pv = (v); \
2b5554c9	138	uatomic_xchg(p, _________pv); \
7e30abe3 MD	139	})
	140
	141
	142	#define _rcu_set_pointer(p, v) \
3daae22a	143	do { \
bdffa73a	144	__typeof__(*p) _________pv = (v); \
7e30abe3 MD	145	if (!__builtin_constant_p(v) \|\| \
7e30abe3 MD	146	((v) != NULL)) \
5481ddb3	147	cmm_wmb(); \
bf9de1b7	148	uatomic_set(p, _________pv); \
3daae22a	149	} while (0)
7e30abe3	150
7e30abe3 MD	151	/**
	152	* _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
	153	* meant to be read by RCU read-side critical sections. Returns the assigned
	154	* value.
	155	*
	156	* Documents which pointers will be dereferenced by RCU read-side critical
	157	* sections and adds the required memory barriers on architectures requiring
	158	* them. It also makes sure the compiler does not reorder code initializing the
	159	* data structure before its publication.
	160	*
edf8de69	161	* Should match rcu_dereference().
a5a9f428 PM	162	*
	163	* This macro is less than 10 lines long. The intent is that this macro
	164	* meets the 10-line criterion in LGPL, allowing this function to be
	165	* expanded directly in non-LGPL code.
7e30abe3	166	*/
7e30abe3 MD	167	#define _rcu_assign_pointer(p, v) _rcu_set_pointer(&(p), v)
7e30abe3 MD	168
67ecffc0	169	#ifdef __cplusplus
36bc70a8 MD	170	}
	171	#endif
	172
7e30abe3	173	#endif /* _URCU_POINTER_STATIC_H */