X-Git-Url: https://git.liburcu.org/?p=urcu.git;a=blobdiff_plain;f=urcu-static.h;h=f819e6fc9aa07e9f4e58329661bf5a494936bb25;hp=c47a759d132357a7af23301ece58c238a200d2ec;hb=4d1ce26fea0296ff67791c881836393fdd020cf9;hpb=adcfce542ec1c28796c3c641021a6d37f97b6f6f diff --git a/urcu-static.h b/urcu-static.h index c47a759..f819e6f 100644 --- a/urcu-static.h +++ b/urcu-static.h @@ -4,14 +4,27 @@ /* * urcu-static.h * - * Userspace RCU header, to be included only in LGPL-compatible code. + * Userspace RCU header. * - * Copyright February 2009 - Mathieu Desnoyers + * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking + * dynamically with the userspace rcu library. * - * Credits for Paul E. McKenney - * for inspiration coming from the Linux kernel RCU and rcu-preempt. + * Copyright (c) 2009 Mathieu Desnoyers + * Copyright (c) 2009 Paul E. McKenney, IBM Corporation. * - * Distributed under LGPLv2.1 + * 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. */ @@ -63,6 +76,15 @@ * Inserts memory barriers on architectures that require them (currently only * Alpha) and documents which pointers are protected by RCU. * + * The compiler memory barrier in 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. + * * Should match rcu_assign_pointer() or rcu_xchg_pointer(). */ @@ -95,6 +117,12 @@ */ #define KICK_READER_LOOPS 10000 +#ifdef DEBUG_RCU +#define rcu_assert(args...) assert(args) +#else +#define rcu_assert(args...) +#endif + #ifdef DEBUG_YIELD #include #include @@ -104,8 +132,11 @@ #define YIELD_READ (1 << 0) #define YIELD_WRITE (1 << 1) -/* Updates without DEBUG_FULL_MB are much slower. Account this in the delay */ -#ifdef DEBUG_FULL_MB +/* + * Updates without URCU_MB are much slower. Account this in + * the delay. + */ +#ifdef URCU_MB /* maximum sleep delay, in us */ #define MAX_SLEEP 50 #else @@ -148,7 +179,7 @@ static inline void debug_yield_init(void) } #endif -#ifdef DEBUG_FULL_MB +#ifdef URCU_MB static inline void reader_barrier() { smp_mb(); @@ -199,22 +230,16 @@ static inline void _rcu_read_lock(void) tmp = urcu_active_readers; /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */ - /* - * The data dependency "read urcu_gp_ctr, write urcu_active_readers", - * serializes those two memory operations. The memory barrier in the - * signal handler ensures we receive the proper memory commit barriers - * required by _STORE_SHARED and _LOAD_SHARED whenever communication - * with the writer is needed. - */ - if (likely(!(tmp & RCU_GP_CTR_NEST_MASK))) + if (likely(!(tmp & RCU_GP_CTR_NEST_MASK))) { _STORE_SHARED(urcu_active_readers, _LOAD_SHARED(urcu_gp_ctr)); - else + /* + * Set active readers count for outermost nesting level before + * accessing the pointer. See force_mb_all_threads(). + */ + reader_barrier(); + } else { _STORE_SHARED(urcu_active_readers, tmp + RCU_GP_COUNT); - /* - * Increment active readers count before accessing the pointer. - * See force_mb_all_threads(). - */ - reader_barrier(); + } } static inline void _rcu_read_unlock(void) @@ -223,6 +248,9 @@ static inline void _rcu_read_unlock(void) /* * Finish using rcu before decrementing the pointer. * See force_mb_all_threads(). + * Formally only needed for outermost nesting level, but leave barrier + * in place for nested unlocks to remove a branch from the common case + * (no nesting). */ _STORE_SHARED(urcu_active_readers, urcu_active_readers - RCU_GP_COUNT); } @@ -248,9 +276,25 @@ static inline void _rcu_read_unlock(void) STORE_SHARED(p, v); \ }) +/** + * _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 !). + */ + +#define _rcu_cmpxchg_pointer(p, old, _new) \ + ({ \ + if (!__builtin_constant_p(_new) || \ + ((_new) != NULL)) \ + wmb(); \ + cmpxchg(p, old, _new); \ + }) + /** * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous - * pointer to the data structure, which can be safely freed after waitin for a + * pointer to the data structure, which can be safely freed after waiting for a * quiescent state using synchronize_rcu(). */