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_testandset_<type>
(
volatile
<type> *p,
u_int 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 accesses 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 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 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 subtraction 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 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
“
_rel
” is inserted into the function
name immediately prior 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 completed 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 include 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)
-
atomic_clear
(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)
-
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)
-
atomic_subtract
(p,
v)
-
atomic_store
(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_testandset
(p,
v)
-
bit = 1 << (v % (sizeof(*p) * NBBY));
tmp = (*p & bit) != 0;
*p |= bit;
return (tmp);
The
atomic_testandset
() 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
() 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.