xref: /freebsd/share/man/man9/atomic.9 (revision eaca618337271234db8f35cf8d69109cee73c969)

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.Dd October 27, 2000
.Os
.Dt ATOMIC 9
.Sh NAME
.Nm atomic_add ,
.Nm atomic_clear ,
.Nm atomic_cmpset ,
.Nm atomic_load ,
.Nm atomic_readandclear ,
.Nm atomic_set ,
.Nm atomic_subtract ,
.Nm atomic_store
.Nd atomic operations
.Sh SYNOPSIS
.Fd #include <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_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"
.Sh 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.
.Ss 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:
.Bl -tag -offset indent -width short
.It char
unsigned character
.It short
unsigned short integer
.It int
unsigned integer
.It long
unsigned long integer
.It ptr
unsigned integer the size of a pointer
.It 8
unsigned 8-bit integer
.It 16
unsigned 16-bit integer
.It 32
unsigned 32-bit integer
.It 64
unsigned 64-bit integer
.El
.Pp
For example, the function to atomically add two integers is called
.Fn atomic_add_int .
.Ss Memory Barriers
Memory barriers are used to guarantee the order the order of data accesses in
two ways.
First, they specify hints to the compiler to not re-order or optimize the
operations.
Secondly, on architectures that do not guarantee ordered data accesses,
special instructions or special variants 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 memory barriers.
The first form just performs the operation without any explicit barriers.
The second form uses a read memory barrier, and the final variant uses a write
memory barrier.
.Pp
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
.Dq _acq
is inserted into the function name immediately prior to the
.Em _type
suffix.
For example, to subtract two integers ensuring that any later writes will
happen after the subtraction is performed, use
.Fn atomic_subtract_acq_int .
.Pp
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 operation is said to have release semantics as it releases
any pending data accesses to be completed before its operation is performed.
To denote this, the suffix
.Dq _rel
is inserted into the function name immediately prior to the
.Em _type
suffix.
For example, to add two long integers ensuring that all previous
writes will happen first, use
.Fn atomic_add_rel_long .
.Pp
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 completed before the lock is released.
.Pp
.Ss 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
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 include the affected memory region.
On the
.Tn i386
architecture, the cache coherency model requires that the hardware perform
this task, thus the atomic operations are atomic across multiple processors.
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
*p += v;
.Ed
.It Fn atomic_clear "p" "v"
.Bd -literal
*p &= ~v;
.Ed
.It Fn atomic_cmpset "dst" "old" "new"
.Bd -literal
if (*dst == old) {
	*dst = new;
	return 1;
} else
	return 0;
.Ed
.El
.Pp
The
.Fn atomic_cmpset
functions are not implemented for the types char, short, 8, and 16.
.Bl -hang
.It Fn atomic_load "addr"
.Bd -literal
return (*addr)
.Ed
.El
.Pp
The
.Fn atomic_load
functions always have acquire semantics.
.Bl -hang
.It Fn atomic_readandclear "addr"
.Bd -literal
temp = *addr;
*addr = 0;
return (temp);
.Ed
.El
.Pp
The
.Fn 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.
.Bl -hang
.It Fn atomic_set "p" "v"
.Bd -literal
*p |= v;
.Ed
.It Fn atomic_subtract "p" "v"
.Bd -literal
*p -= v;
.Ed
.It Fn atomic_store "p" "v"
.Bd -literal
*p = v;
.Ed
.El
.Pp
The
.Fn atomic_store
functions always have release semantics.
.Pp
The type
.Dq 64
is currently not implemented for any of the atomic operations on the
.Tn i386
architecture.
.Sh RETURN VALUES
.Fn atomic_cmpset
returns the result of the compare operation.
.Fn atomic_load
and
.Fn atomic_readandclear
return the value at the specified address.
.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
.Li struct mtx
is a pointer, the
.Dq ptr
type is used.
.Bd -literal
#define _obtain_lock(mp, tid)						\\
	atomic_cmpset_acq_ptr(&(mp)->mtx_lock, (void *)MTX_UNOWNED, (tid))

/* Get a sleep lock, deal with recursion inline. */
#define	_getlock_sleep(mp, tid, type) do {				\\
	if (!_obtain_lock(mp, tid)) {					\\
		if (((mp)->mtx_lock & MTX_FLAGMASK) != ((uintptr_t)(tid)))\\
			mtx_enter_hard(mp, (type) & MTX_HARDOPTS, 0);	\\
		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 suppored the types char, short, int, and long.
The
.Fn atomic_cmpset ,
.Fn atomic_load ,
.Fn atomic_readandclear ,
and
.Fn atomic_store
operations were added in
.Fx 5.0 .
The types 8, 16, 32, 64, and ptr and all of the acquire and release variants
were added in
.Fx 5.0
as well.