Provided by: freebsd-manpages_11.1-3_all 
NAME
atomic_add, atomic_clear, atomic_cmpset, atomic_fcmpset, 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);
int
atomic_fcmpset_[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_fcmpset(dst, *old, new)
On architectures implementing Compare And Swap operation in hardware, the functionality can
be described as
if (*dst == *old) {
*dst = new;
return (1);
} else {
*old = *dst;
return (0);
}
On architectures which provide Load Linked/Store Conditional primitive, the write to *dst
might also fail for several reasons, most important of which is a parallel write to *dst
cache line by other CPU. In this case atomic_fcmpset() function also returns false, despite
*old == *dst.
The atomic_fcmpset() 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_fcmpset() function returns true if the operation succeeded. Otherwise it returns
false and sets *old to the found value. 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.