/*
* Copyright (C) 2007 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef ANDROID_CUTILS_ATOMIC_H
#define ANDROID_CUTILS_ATOMIC_H
#include <stdint.h>
#include <sys/types.h>
#include <stdatomic.h>
#ifndef ANDROID_ATOMIC_INLINE
#define ANDROID_ATOMIC_INLINE static inline
#endif
/*
* A handful of basic atomic operations.
* THESE ARE HERE FOR LEGACY REASONS ONLY. AVOID.
*
* PREFERRED ALTERNATIVES:
* - Use C++/C/pthread locks/mutexes whenever there is not a
* convincing reason to do otherwise. Note that very clever and
* complicated, but correct, lock-free code is often slower than
* using locks, especially where nontrivial data structures
* are involved.
* - C11 stdatomic.h.
* - Where supported, C++11 std::atomic<T> .
*
* PLEASE STOP READING HERE UNLESS YOU ARE TRYING TO UNDERSTAND
* OR UPDATE OLD CODE.
*
* The "acquire" and "release" terms can be defined intuitively in terms
* of the placement of memory barriers in a simple lock implementation:
* - wait until compare-and-swap(lock-is-free --> lock-is-held) succeeds
* - barrier
* - [do work]
* - barrier
* - store(lock-is-free)
* In very crude terms, the initial (acquire) barrier prevents any of the
* "work" from happening before the lock is held, and the later (release)
* barrier ensures that all of the work happens before the lock is released.
* (Think of cached writes, cache read-ahead, and instruction reordering
* around the CAS and store instructions.)
*
* The barriers must apply to both the compiler and the CPU. Note it is
* legal for instructions that occur before an "acquire" barrier to be
* moved down below it, and for instructions that occur after a "release"
* barrier to be moved up above it.
*
* The ARM-driven implementation we use here is short on subtlety,
* and actually requests a full barrier from the compiler and the CPU.
* The only difference between acquire and release is in whether they
* are issued before or after the atomic operation with which they
* are associated. To ease the transition to C/C++ atomic intrinsics,
* you should not rely on this, and instead assume that only the minimal
* acquire/release protection is provided.
*
* NOTE: all int32_t* values are expected to be aligned on 32-bit boundaries.
* If they are not, atomicity is not guaranteed.
*/
/*
* Basic arithmetic and bitwise operations. These all provide a
* barrier with "release" ordering, and return the previous value.
*
* These have the same characteristics (e.g. what happens on overflow)
* as the equivalent non-atomic C operations.
*/
ANDROID_ATOMIC_INLINE
int32_t android_atomic_inc(volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
/* Int32_t, if it exists, is the same as int_least32_t. */
return atomic_fetch_add_explicit(a, 1, memory_order_release);
}
ANDROID_ATOMIC_INLINE
int32_t android_atomic_dec(volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return atomic_fetch_sub_explicit(a, 1, memory_order_release);
}
ANDROID_ATOMIC_INLINE
int32_t android_atomic_add(int32_t value, volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return atomic_fetch_add_explicit(a, value, memory_order_release);
}
ANDROID_ATOMIC_INLINE
int32_t android_atomic_and(int32_t value, volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return atomic_fetch_and_explicit(a, value, memory_order_release);
}
ANDROID_ATOMIC_INLINE
int32_t android_atomic_or(int32_t value, volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return atomic_fetch_or_explicit(a, value, memory_order_release);
}
/*
* Perform an atomic load with "acquire" or "release" ordering.
*
* Note that the notion of a "release" ordering for a load does not
* really fit into the C11 or C++11 memory model. The extra ordering
* is normally observable only by code using memory_order_relaxed
* atomics, or data races. In the rare cases in which such ordering
* is called for, use memory_order_relaxed atomics and a leading
* atomic_thread_fence (typically with memory_order_acquire,
* not memory_order_release!) instead. If you do not understand
* this comment, you are in the vast majority, and should not be
* using release loads or replacing them with anything other than
* locks or default sequentially consistent atomics.
*/
ANDROID_ATOMIC_INLINE
int32_t android_atomic_acquire_load(volatile const int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return atomic_load_explicit(a, memory_order_acquire);
}
ANDROID_ATOMIC_INLINE
int32_t android_atomic_release_load(volatile const int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
atomic_thread_fence(memory_order_seq_cst);
/* Any reasonable clients of this interface would probably prefer */
/* something weaker. But some remaining clients seem to be */
/* abusing this API in strange ways, e.g. by using it as a fence. */
/* Thus we are conservative until we can get rid of remaining */
/* clients (and this function). */
return atomic_load_explicit(a, memory_order_relaxed);
}
/*
* Perform an atomic store with "acquire" or "release" ordering.
*
* Note that the notion of an "acquire" ordering for a store does not
* really fit into the C11 or C++11 memory model. The extra ordering
* is normally observable only by code using memory_order_relaxed
* atomics, or data races. In the rare cases in which such ordering
* is called for, use memory_order_relaxed atomics and a trailing
* atomic_thread_fence (typically with memory_order_release,
* not memory_order_acquire!) instead.
*/
ANDROID_ATOMIC_INLINE
void android_atomic_acquire_store(int32_t value, volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
atomic_store_explicit(a, value, memory_order_relaxed);
atomic_thread_fence(memory_order_seq_cst);
/* Again overly conservative to accomodate weird clients. */
}
ANDROID_ATOMIC_INLINE
void android_atomic_release_store(int32_t value, volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
atomic_store_explicit(a, value, memory_order_release);
}
/*
* Compare-and-set operation with "acquire" or "release" ordering.
*
* This returns zero if the new value was successfully stored, which will
* only happen when *addr == oldvalue.
*
* (The return value is inverted from implementations on other platforms,
* but matches the ARM ldrex/strex result.)
*
* Implementations that use the release CAS in a loop may be less efficient
* than possible, because we re-issue the memory barrier on each iteration.
*/
ANDROID_ATOMIC_INLINE
int android_atomic_acquire_cas(int32_t oldvalue, int32_t newvalue,
volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return (int)(!atomic_compare_exchange_strong_explicit(
a, &oldvalue, newvalue,
memory_order_acquire,
memory_order_acquire));
}
ANDROID_ATOMIC_INLINE
int android_atomic_release_cas(int32_t oldvalue, int32_t newvalue,
volatile int32_t* addr)
{
volatile atomic_int_least32_t* a = (volatile atomic_int_least32_t*)addr;
return (int)(!atomic_compare_exchange_strong_explicit(
a, &oldvalue, newvalue,
memory_order_release,
memory_order_relaxed));
}
/*
* Fence primitives.
*/
ANDROID_ATOMIC_INLINE
void android_compiler_barrier(void)
{
__asm__ __volatile__ ("" : : : "memory");
/* Could probably also be: */
/* atomic_signal_fence(memory_order_seq_cst); */
}
ANDROID_ATOMIC_INLINE
void android_memory_barrier(void)
{
atomic_thread_fence(memory_order_seq_cst);
}
/*
* Aliases for code using an older version of this header. These are now
* deprecated and should not be used. The definitions will be removed
* in a future release.
*/
#define android_atomic_write android_atomic_release_store
#define android_atomic_cmpxchg android_atomic_release_cas
#endif // ANDROID_CUTILS_ATOMIC_H