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.Dd August 14, 2015
.Dt ATOMIC 9
.Os
.Sh NAME
.Nm atomic_add ,
.Nm atomic_clear ,
.Nm atomic_cmpset ,
.Nm atomic_fetchadd ,
.Nm atomic_load ,
.Nm atomic_readandclear ,
.Nm atomic_set ,
.Nm atomic_subtract ,
.Nm atomic_store
.Nd atomic operations
.Sh SYNOPSIS
.In sys/types.h
.In machine/atomic.h
.Ft void
.Fn atomic_add_[acq_|rel_]<type> "volatile <type> *p" "<type> v"
.Ft void
.Fn atomic_clear_[acq_|rel_]<type> "volatile <type> *p" "<type> v"
.Ft int
.Fo atomic_cmpset_[acq_|rel_]<type>
.Fa "volatile <type> *dst"
.Fa "<type> old"
.Fa "<type> new"
.Fc
.Ft <type>
.Fn atomic_fetchadd_<type> "volatile <type> *p" "<type> v"
.Ft <type>
.Fn atomic_load_acq_<type> "volatile <type> *p"
.Ft <type>
.Fn atomic_readandclear_<type> "volatile <type> *p"
.Ft void
.Fn atomic_set_[acq_|rel_]<type> "volatile <type> *p" "<type> v"
.Ft void
.Fn atomic_subtract_[acq_|rel_]<type> "volatile <type> *p" "<type> v"
.Ft void
.Fn atomic_store_rel_<type> "volatile <type> *p" "<type> v"
.Ft <type>
.Fn atomic_swap_<type> "volatile <type> *p" "<type> v"
.Ft int
.Fn atomic_testandset_<type> "volatile <type> *p" "u_int v"
.Sh 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.
.Ss Types
Each atomic operation operates on a specific
.Fa type .
The type to use is indicated in the function name.
The available types that can be used are:
.Pp
.Bl -tag -offset indent -width short -compact
.It Li int
unsigned integer
.It Li long
unsigned long integer
.It Li ptr
unsigned integer the size of a pointer
.It Li 32
unsigned 32-bit integer
.It Li 64
unsigned 64-bit integer
.El
.Pp
For example, the function to atomically add two integers is called
.Fn atomic_add_int .
.Pp
Certain architectures also provide operations for types smaller than
.Dq Li int .
.Pp
.Bl -tag -offset indent -width short -compact
.It Li char
unsigned character
.It Li short
unsigned short integer
.It Li 8
unsigned 8-bit integer
.It Li 16
unsigned 16-bit integer
.El
.Pp
These must not be used in MI code because the instructions to implement them
efficiently might not be available.
.Ss Acquire and Release Operations
By default, a thread's accesses to different memory locations might not be
performed in
.Em 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
.Fx
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
.Em acquire
and
.Em release
semantics.
.Pp
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.
.Pp
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
.Dq Li _acq
is inserted into the function name immediately prior to the
.Dq Li _ Ns Aq Fa type
suffix.
For example, to subtract two integers ensuring that subsequent loads and
stores happen after the subtraction is performed, use
.Fn atomic_subtract_acq_int .
.Pp
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
.Dq Li _rel
is inserted into the function name immediately prior to the
.Dq Li _ Ns Aq Fa type
suffix.
For example, to add two long integers ensuring that all prior loads and
stores happen before the addition, use
.Fn atomic_add_rel_long .
.Pp
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.
.Ss 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,
.Dv VM_MEMATTR_DEFAULT ,
is guaranteed by all architectures that are supported by
.Fx .
For example, cache coherence is guaranteed on write-back memory by the
.Tn amd64
and
.Tn 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.
On the
.Tn ia64
architecture, coherency is only guaranteed for pages that are configured to
using a caching policy of either uncached or write back.
.Ss Semantics
This section describes the semantics of each operation using a C like notation.
.Bl -hang
.It Fn atomic_add p v
.Bd -literal -compact
*p += v;
.Ed
.It Fn atomic_clear p v
.Bd -literal -compact
*p &= ~v;
.Ed
.It Fn atomic_cmpset dst old new
.Bd -literal -compact
if (*dst == old) {
	*dst = new;
	return (1);
} else
	return (0);
.Ed
.El
.Pp
The
.Fn atomic_cmpset
functions are not implemented for the types
.Dq Li char ,
.Dq Li short ,
.Dq Li 8 ,
and
.Dq Li 16 .
.Bl -hang
.It Fn atomic_fetchadd p v
.Bd -literal -compact
tmp = *p;
*p += v;
return (tmp);
.Ed
.El
.Pp
The
.Fn atomic_fetchadd
functions are only implemented for the types
.Dq Li int ,
.Dq Li long
and
.Dq Li 32
and do not have any variants with memory barriers at this time.
.Bl -hang
.It Fn atomic_load p
.Bd -literal -compact
return (*p);
.Ed
.El
.Pp
The
.Fn atomic_load
functions are only provided with acquire memory barriers.
.Bl -hang
.It Fn atomic_readandclear p
.Bd -literal -compact
tmp = *p;
*p = 0;
return (tmp);
.Ed
.El
.Pp
The
.Fn atomic_readandclear
functions are not implemented for the types
.Dq Li char ,
.Dq Li short ,
.Dq Li ptr ,
.Dq Li 8 ,
and
.Dq Li 16
and do not have any variants with memory barriers at this time.
.Bl -hang
.It Fn atomic_set p v
.Bd -literal -compact
*p |= v;
.Ed
.It Fn atomic_subtract p v
.Bd -literal -compact
*p -= v;
.Ed
.It Fn atomic_store p v
.Bd -literal -compact
*p = v;
.Ed
.El
.Pp
The
.Fn atomic_store
functions are only provided with release memory barriers.
.Bl -hang
.It Fn atomic_swap p v
.Bd -literal -compact
tmp = *p;
*p = v;
return (tmp);
.Ed
.El
.Pp
The
.Fn atomic_swap
functions are not implemented for the types
.Dq Li char ,
.Dq Li short ,
.Dq Li ptr ,
.Dq Li 8 ,
and
.Dq Li 16
and do not have any variants with memory barriers at this time.
.Bl -hang
.It Fn atomic_testandset p v
.Bd -literal -compact
bit = 1 << (v % (sizeof(*p) * NBBY));
tmp = (*p & bit) != 0;
*p |= bit;
return (tmp);
.Ed
.El
.Pp
The
.Fn atomic_testandset
functions are only implemented for the types
.Dq Li int ,
.Dq Li long
and
.Dq Li 32
and do not have any variants with memory barriers at this time.
.Pp
The type
.Dq Li 64
is currently not implemented for any of the atomic operations on the
.Tn arm ,
.Tn i386 ,
and
.Tn powerpc
architectures.
.Sh RETURN VALUES
The
.Fn atomic_cmpset
function returns the result of the compare operation.
The
.Fn atomic_fetchadd ,
.Fn atomic_load ,
.Fn atomic_readandclear ,
and
.Fn atomic_swap
functions return the value at the specified address.
The
.Fn atomic_testandset
function returns the result of the test operation.
.Sh EXAMPLES
This example uses the
.Fn atomic_cmpset_acq_ptr
and
.Fn atomic_set_ptr
functions to obtain a sleep mutex and handle recursion.
Since the
.Va mtx_lock
member of a
.Vt "struct mtx"
is a pointer, the
.Dq Li ptr
type is used.
.Bd -literal
/* 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)
.Ed
.Sh HISTORY
The
.Fn atomic_add ,
.Fn atomic_clear ,
.Fn atomic_set ,
and
.Fn atomic_subtract
operations were first introduced in
.Fx 3.0 .
This first set only supported the types
.Dq Li char ,
.Dq Li short ,
.Dq Li int ,
and
.Dq Li long .
The
.Fn atomic_cmpset ,
.Fn atomic_load ,
.Fn atomic_readandclear ,
and
.Fn atomic_store
operations were added in
.Fx 5.0 .
The types
.Dq Li 8 ,
.Dq Li 16 ,
.Dq Li 32 ,
.Dq Li 64 ,
and
.Dq Li ptr
and all of the acquire and release variants
were added in
.Fx 5.0
as well.
The
.Fn atomic_fetchadd
operations were added in
.Fx 6.0 .
The
.Fn atomic_swap
and
.Fn atomic_testandset
operations were added in
.Fx 10.0 .