redis/src/atomicvar.h

/* This file implements atomic counters using c11 _Atomic, __atomic or __sync
 * macros if available, otherwise we will throw an error when compile.
 *
 * The exported interface is composed of the following macros:
 *
 * atomicIncr(var,count) -- Increment the atomic counter
 * atomicGetIncr(var,oldvalue_var,count) -- Get and increment the atomic counter
 * atomicIncrGet(var,newvalue_var,count) -- Increment and get the atomic counter new value
 * atomicDecr(var,count) -- Decrement the atomic counter
 * atomicGet(var,dstvar) -- Fetch the atomic counter value
 * atomicSet(var,value)  -- Set the atomic counter value
 * atomicGetWithSync(var,value)  -- 'atomicGet' with inter-thread synchronization
 * atomicSetWithSync(var,value)  -- 'atomicSet' with inter-thread synchronization
 * 
 * Atomic operations on flags. 
 * Flag type can be int, long, long long or their unsigned counterparts.
 * The value of the flag can be 1 or 0.
 * 
 * atomicFlagGetSet(var,oldvalue_var) -- Get and set the atomic counter value
 * 
 * NOTE1: __atomic* and _Atomic implementations can be actually elaborated to support any value by changing the 
 * hardcoded new value passed to __atomic_exchange* from 1 to @param count
 * i.e oldvalue_var = atomic_exchange_explicit(&var, count).
 * However, in order to be compatible with the __sync functions family, we can use only 0 and 1.
 * The only exchange alternative suggested by __sync is __sync_lock_test_and_set, 
 * But as described by the gnu manual for __sync_lock_test_and_set():
 * https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
 * "A target may support reduced functionality here by which the only valid value to store is the immediate constant 1. The exact value
 * actually stored in *ptr is implementation defined."
 * Hence, we can't rely on it for a any value other than 1.
 * We eventually chose to implement this method with __sync_val_compare_and_swap since it satisfies functionality needed for atomicFlagGetSet
 * (if the flag was 0 -> set to 1, if it's already 1 -> do nothing, but the final result is that the flag is set), 
 * and also it has a full barrier (__sync_lock_test_and_set has acquire barrier).
 * 
 * NOTE2: Unlike other atomic type, which aren't guaranteed to be lock free, c11 atmoic_flag does.
 * To check whether a type is lock free, atomic_is_lock_free() can be used. 
 * It can be considered to limit the flag type to atomic_flag to improve performance.
 * 
 * Never use return value from the macros, instead use the AtomicGetIncr()
 * if you need to get the current value and increment it atomically, like
 * in the following example:
 *
 *  long oldvalue;
 *  atomicGetIncr(myvar,oldvalue,1);
 *  doSomethingWith(oldvalue);
 *
 * ----------------------------------------------------------------------------
 *
 * Copyright (c) 2015-Present, Redis Ltd.
 * All rights reserved.
 *
 * Licensed under your choice of the Redis Source Available License 2.0
 * (RSALv2) or the Server Side Public License v1 (SSPLv1).
 */

#include <pthread.h>
#include "config.h"

#ifndef __ATOMIC_VAR_H
#define __ATOMIC_VAR_H

/* Define redisAtomic for atomic variable. */
#define redisAtomic

/* To test Redis with Helgrind (a Valgrind tool) it is useful to define
 * the following macro, so that __sync macros are used: those can be detected
 * by Helgrind (even if they are less efficient) so that no false positive
 * is reported. */
// #define __ATOMIC_VAR_FORCE_SYNC_MACROS

/* There will be many false positives if we test Redis with Helgrind, since
 * Helgrind can't understand we have imposed ordering on the program, so
 * we use macros in helgrind.h to tell Helgrind inter-thread happens-before
 * relationship explicitly for avoiding false positives.
 *
 * For more details, please see: valgrind/helgrind.h and
 * https://www.valgrind.org/docs/manual/hg-manual.html#hg-manual.effective-use
 *
 * These macros take effect only when 'make helgrind', and you must first
 * install Valgrind in the default path configuration. */
#ifdef __ATOMIC_VAR_FORCE_SYNC_MACROS
#include <valgrind/helgrind.h>
#else
#define ANNOTATE_HAPPENS_BEFORE(v) ((void) v)
#define ANNOTATE_HAPPENS_AFTER(v)  ((void) v)
#endif

#if !defined(__ATOMIC_VAR_FORCE_SYNC_MACROS) && defined(__STDC_VERSION__) && \
    (__STDC_VERSION__ >= 201112L) && !defined(__STDC_NO_ATOMICS__)
/* Use '_Atomic' keyword if the compiler supports. */
#undef  redisAtomic
#define redisAtomic _Atomic
/* Implementation using _Atomic in C11. */

#include <stdatomic.h>
#define atomicIncr(var,count) atomic_fetch_add_explicit(&var,(count),memory_order_relaxed)
#define atomicGetIncr(var,oldvalue_var,count) do { \
    oldvalue_var = atomic_fetch_add_explicit(&var,(count),memory_order_relaxed); \
} while(0)
#define atomicIncrGet(var, newvalue_var, count) \
    newvalue_var = atomicIncr(var,count) + count
#define atomicDecr(var,count) atomic_fetch_sub_explicit(&var,(count),memory_order_relaxed)
#define atomicGet(var,dstvar) do { \
    dstvar = atomic_load_explicit(&var,memory_order_relaxed); \
} while(0)
#define atomicSet(var,value) atomic_store_explicit(&var,value,memory_order_relaxed)
#define atomicGetWithSync(var,dstvar) do { \
    dstvar = atomic_load_explicit(&var,memory_order_seq_cst); \
} while(0)
#define atomicSetWithSync(var,value) \
    atomic_store_explicit(&var,value,memory_order_seq_cst)
#define atomicFlagGetSet(var,oldvalue_var) \
    oldvalue_var = atomic_exchange_explicit(&var,1,memory_order_relaxed)
#define REDIS_ATOMIC_API "c11-builtin"

#elif !defined(__ATOMIC_VAR_FORCE_SYNC_MACROS) && \
    (!defined(__clang__) || !defined(__APPLE__) || __apple_build_version__ > 4210057) && \
    defined(__ATOMIC_RELAXED) && defined(__ATOMIC_SEQ_CST)
/* Implementation using __atomic macros. */

#define atomicIncr(var,count) __atomic_add_fetch(&var,(count),__ATOMIC_RELAXED)
#define atomicIncrGet(var, newvalue_var, count) \
    newvalue_var = __atomic_add_fetch(&var,(count),__ATOMIC_RELAXED)
#define atomicGetIncr(var,oldvalue_var,count) do { \
    oldvalue_var = __atomic_fetch_add(&var,(count),__ATOMIC_RELAXED); \
} while(0)
#define atomicDecr(var,count) __atomic_sub_fetch(&var,(count),__ATOMIC_RELAXED)
#define atomicGet(var,dstvar) do { \
    dstvar = __atomic_load_n(&var,__ATOMIC_RELAXED); \
} while(0)
#define atomicSet(var,value) __atomic_store_n(&var,value,__ATOMIC_RELAXED)
#define atomicGetWithSync(var,dstvar) do { \
    dstvar = __atomic_load_n(&var,__ATOMIC_SEQ_CST); \
} while(0)
#define atomicSetWithSync(var,value) \
    __atomic_store_n(&var,value,__ATOMIC_SEQ_CST)
#define atomicFlagGetSet(var,oldvalue_var) \
    oldvalue_var = __atomic_exchange_n(&var,1,__ATOMIC_RELAXED)
#define REDIS_ATOMIC_API "atomic-builtin"

#elif defined(HAVE_ATOMIC)
/* Implementation using __sync macros. */

#define atomicIncr(var,count) __sync_add_and_fetch(&var,(count))
#define atomicIncrGet(var, newvalue_var, count) \
    newvalue_var = __sync_add_and_fetch(&var,(count))
#define atomicGetIncr(var,oldvalue_var,count) do { \
    oldvalue_var = __sync_fetch_and_add(&var,(count)); \
} while(0)
#define atomicDecr(var,count) __sync_sub_and_fetch(&var,(count))
#define atomicGet(var,dstvar) do { \
    dstvar = __sync_sub_and_fetch(&var,0); \
} while(0)
#define atomicSet(var,value) do { \
    while(!__sync_bool_compare_and_swap(&var,var,value)); \
} while(0)
/* Actually the builtin issues a full memory barrier by default. */
#define atomicGetWithSync(var,dstvar) do { \
    dstvar = __sync_sub_and_fetch(&var,0,__sync_synchronize); \
    ANNOTATE_HAPPENS_AFTER(&var); \
} while(0)
#define atomicSetWithSync(var,value) do { \
    ANNOTATE_HAPPENS_BEFORE(&var);  \
    while(!__sync_bool_compare_and_swap(&var,var,value,__sync_synchronize)); \
} while(0)
#define atomicFlagGetSet(var,oldvalue_var) \
    oldvalue_var = __sync_val_compare_and_swap(&var,0,1)
#define REDIS_ATOMIC_API "sync-builtin"

#else
#error "Unable to determine atomic operations for your platform"

#endif
#endif /* __ATOMIC_VAR_H */