ATOMIC(9)

NAME
atomic_add, atomic_clear, atomic_cmpset, atomic_fetchadd, atomic_load, atomic_readandclear, atomic_set, atomic_subtract, atomic_store - atomic operations

SYNOPSIS
#include <sys/types.h>
#include <machine/atomic.h>
void
atomic_add_[acq_|rel_]<type>(volatile <type> *p, <type> v);
void
atomic_clear_[acq_|rel_]<type>(volatile <type> *p, <type> v);
int
atomic_cmpset_[acq_|rel_]<type>(volatile <type> *dst, <type> old, <type> new);
<type>
atomic_fetchadd_<type>(volatile <type> *p, <type> v);
<type>
atomic_load_acq_<type>(volatile <type> *p);
<type>
atomic_readandclear_<type>(volatile <type> *p);
void
atomic_set_[acq_|rel_]<type>(volatile <type> *p, <type> v);
void
atomic_subtract_[acq_|rel_]<type>(volatile <type> *p, <type> v);
void
atomic_store_rel_<type>(volatile <type> *p, <type> v);
<type>
atomic_swap_<type>(volatile <type> *p, <type> v);
int
atomic_testandclear_<type>(volatile <type> *p, u_int v);
int
atomic_testandset_<type>(volatile <type> *p, u_int v);

DESCRIPTION
Each of the atomic operations is guaranteed to be atomic across multiple threads and in the presence of interrupts. They can be used to implement reference counts or as building blocks for more advanced synchronization primitives such as mutexes.

  Types
Each atomic operation operates on a specific type. The type to use is indicated in the function name. The available types that can be used are:

int unsigned integer
long
    unsigned long integer
ptr unsigned integer the size of a pointer
32  unsigned 32-bit integer
64  unsigned 64-bit integer

For example, the function to atomically add two integers is called atomic_add_int().

Certain architectures also provide operations for types smaller than “int”.

char
    unsigned character
short
    unsigned short integer
8   unsigned 8-bit integer
16  unsigned 16-bit integer

These must not be used in MI code because the instructions to implement them efficiently might not be available.

  Acquire and Release Operations
By default, a thread's accesses to different memory locations might not be performed in program order, that is, the order in which the accesses appear in the source code. To optimize the program's execution, both the compiler and processor might reorder the thread's accesses. However, both ensure that their reordering of the accesses is not visible to the thread. Otherwise, the traditional memory model that is expected by single-threaded programs would be violated. Nonetheless, other threads in a multithreaded program, such as the FreeBSD kernel, might observe the reordering. Moreover, in some cases, such as the implementation of synchronization between threads, arbitrary reordering might result in the incorrect execution of the program. To constrain the reordering that both the compiler and processor might perform on a thread's accesses, the thread should use atomic operations with acquire and release semantics.

Most of the atomic operations on memory have three variants. The first variant performs the operation without imposing any ordering constraints on memory accesses to other locations. The second variant has acquire semantics, and the third variant has release semantics. In effect, operations with acquire and release semantics establish one-way barriers to reordering.

When an atomic operation has acquire semantics, the effects of the operation must have completed before any subsequent load or store (by program order) is performed. Conversely, acquire semantics do not require that prior loads or stores have completed before the atomic operation is performed. To denote acquire semantics, the suffix “_acq” is inserted into the function name immediately prior to the “_<type>” suffix. For example, to subtract two integers ensuring that subsequent loads and stores happen after the subtraction is performed, use atomic_subtract_acq_int().

When an atomic operation has release semantics, the effects of all prior loads or stores (by program order) must have completed before the operation is performed. Conversely, release semantics do not require that the effects of the atomic operation must have completed before any subsequent load or store is performed. To denote release semantics, the suffix “_rel” is inserted into the function name immediately prior to the “_<type>” suffix. For example, to add two long integers ensuring that all prior loads and stores happen before the addition, use atomic_add_rel_long().

The one-way barriers provided by acquire and release operations allow the implementations of common synchronization primitives to express their ordering requirements without also imposing unnecessary ordering. For example, for a critical section guarded by a mutex, an acquire operation when the mutex is locked and a release operation when the mutex is unlocked will prevent any loads or stores from moving outside of the critical section. However, they will not prevent the compiler or processor from moving loads or stores into the critical section, which does not violate the semantics of a mutex.

  Multiple Processors
In multiprocessor systems, the atomicity of the atomic operations on memory depends on support for cache coherence in the underlying architecture. In general, cache coherence on the default memory type, VM_MEMATTR_DEFAULT, is guaranteed by all architectures that are supported by FreeBSD. For example, cache coherence is guaranteed on write-back memory by the amd64 and i386 architectures. However, on some architectures, cache coherence might not be enabled on all memory types. To determine if cache coherence is enabled for a non-default memory type, consult the architecture's documentation.

  Semantics
This section describes the semantics of each operation using a C like notation.

atomic_add(p, v)
        *p += v;

atomic_clear(p, v)
        *p &= ~v;

atomic_cmpset(dst, old, new)
        if (*dst == old) {
        	*dst = new;
        	return (1);
        } else
        	return (0);

The atomic_cmpset() functions are not implemented for the types “char”, “short”, “8”, and “16”.

atomic_fetchadd(p, v)
        tmp = *p;
        *p += v;
        return (tmp);

The atomic_fetchadd() functions are only implemented for the types “int”, “long” and “32” and do not have any variants with memory barriers at this time.

atomic_load(p)
        return (*p);

The atomic_load() functions are only provided with acquire memory barriers.

atomic_readandclear(p)
        tmp = *p;
        *p = 0;
        return (tmp);

The atomic_readandclear() functions are not implemented for the types “char”, “short”, “ptr”, “8”, and “16” and do not have any variants with memory barriers at this time.

atomic_set(p, v)
        *p |= v;

atomic_subtract(p, v)
        *p -= v;

atomic_store(p, v)
        *p = v;

The atomic_store() functions are only provided with release memory barriers.

atomic_swap(p, v)
        tmp = *p;
        *p = v;
        return (tmp);

The atomic_swap() functions are not implemented for the types “char”, “short”, “ptr”, “8”, and “16” and do not have any variants with memory barriers at this time.

atomic_testandclear(p, v)
        bit = 1 << (v % (sizeof(*p) * NBBY));
        tmp = (*p & bit) != 0;
        *p &= ~bit;
        return (tmp);

atomic_testandset(p, v)
        bit = 1 << (v % (sizeof(*p) * NBBY));
        tmp = (*p & bit) != 0;
        *p |= bit;
        return (tmp);

The atomic_testandset() and atomic_testandclear() functions are only implemented for the types “int”, “long” and “32” and do not have any variants with memory barriers at this time.

The type “64” is currently not implemented for any of the atomic operations on the arm, i386, and powerpc architectures.

RETURN VALUES
The atomic_cmpset() function returns the result of the compare operation. The atomic_fetchadd(), atomic_load(), atomic_readandclear(), and atomic_swap() functions return the value at the specified address. The atomic_testandset() and atomic_testandclear() function returns the result of the test operation.

EXAMPLES
This example uses the atomic_cmpset_acq_ptr() and atomic_set_ptr() functions to obtain a sleep mutex and handle recursion. Since the mtx_lock member of a struct mtx is a pointer, the “ptr” type is used.

    /* Try to obtain mtx_lock once. */
    #define _obtain_lock(mp, tid)						\
    	atomic_cmpset_acq_ptr(&(mp)->mtx_lock, MTX_UNOWNED, (tid))
    /* Get a sleep lock, deal with recursion inline. */
    #define _get_sleep_lock(mp, tid, opts, file, line) do {			\
    	uintptr_t _tid = (uintptr_t)(tid);				\
    									\
    	if (!_obtain_lock(mp, tid)) {					\
    		if (((mp)->mtx_lock & MTX_FLAGMASK) != _tid)		\
    			_mtx_lock_sleep((mp), _tid, (opts), (file), (line));\
    		else {							\
    			atomic_set_ptr(&(mp)->mtx_lock, MTX_RECURSE);	\
    			(mp)->mtx_recurse++;				\
    		}							\
    	}								\
    } while (0)

HISTORY
The atomic_add(), atomic_clear(), atomic_set(), and atomic_subtract() operations were first introduced in FreeBSD 3.0. This first set only supported the types “char”, “short”, “int”, and “long”. The atomic_cmpset(), atomic_load(), atomic_readandclear(), and atomic_store() operations were added in FreeBSD 5.0. The types “8”, “16”, “32”, “64”, and “ptr” and all of the acquire and release variants were added in FreeBSD 5.0 as well. The atomic_fetchadd() operations were added in FreeBSD 6.0. The atomic_swap() and atomic_testandset() operations were added in FreeBSD 10.0. atomic_testandclear() operation was added in FreeBSD 11.0.