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);

DESCRIPTION

Each of the atomic operations is guaranteed to be atomic 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 may not be available.

Memory Barriers

Memory barriers are used to guarantee the order of data ac

cesses in two
ways. First, they specify hints to the compiler to not re

order or optimize the operations. Second, on architectures that do not

guarantee
ordered data accesses, special instructions or special vari

ants of
instructions are used to indicate to the processor that data

accesses
need to occur in a certain order. As a result, most of the

atomic operations have three variants in order to include optional memo

ry barriers.
The first form just performs the operation without any ex

plicit barriers.
The second form uses a read memory barrier, and the third

variant uses a
write memory barrier.

The second variant of each operation includes a read memory

barrier.
This barrier ensures that the effects of this operation are

completed
before the effects of any later data accesses. As a result,

the operation is said to have acquire semantics as it acquires a

pseudo-lock
requiring further operations to wait until it has completed.

To denote
this, the suffix ``_acq'' is inserted into the function name

immediately
prior to the ``_<type>'' suffix. For example, to subtract

two integers
ensuring that any later writes will happen after the sub

traction is performed, use atomic_subtract_acq_int().

The third variant of each operation includes a write memory

barrier.
This ensures that all effects of all previous data accesses

are completed
before this operation takes place. As a result, the opera

tion is said to
have release semantics as it releases any pending data ac

cesses to be
completed before its operation is performed. To denote

this, the suffix
``_rel'' is inserted into the function name immediately pri

or to the
``_<type>'' suffix. For example, to add two long integers

ensuring that
all previous writes will happen first, use

atomic_add_rel_long().

A practical example of using memory barriers is to ensure

that data
accesses that are protected by a lock are all performed

while the lock is
held. To achieve this, one would use a read barrier when

acquiring the
lock to guarantee that the lock is held before any protected

operations
are performed. Finally, one would use a write barrier when

releasing the
lock to ensure that all of the protected operations are com

pleted before
the lock is released.

Multiple Processors

The current set of atomic operations do not necessarily

guarantee atomicity across multiple processors. To guarantee atomicity

across processors, not only does the individual operation need to be

atomic on the
processor performing the operation, but the result of the

operation needs
to be pushed out to stable storage and the caches of all

other processors
on the system need to invalidate any cache lines that in

clude the
affected memory region. On the i386 architecture, the cache

coherency
model requires that the hardware perform this task, thus the

atomic operations are atomic across multiple processors. On the ia64

architecture,
coherency is only guaranteed for pages that are configured

to using a
caching policy of either uncached or write back.

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'' and ``32'' and do not have any variants with memory

barriers at
this time.

atomic_load(addr)

return (*addr)

The atomic_load() functions always have acquire semantics.

atomic_readandclear(addr)

temp = *addr;
*addr = 0;
return (temp);

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 always have release semantics.

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 com

pare operation.
The atomic_fetchadd(), atomic_load(), and

atomic_readandclear() functions
return the value at the specified address.

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 vari

ants were
added in FreeBSD 5.0 as well. The atomic_fetchadd() opera

tions were
added in FreeBSD 6.0.

BSD October 27, 2000

BSD