atomic_add, atomic_clear, atomic_cmpset, atomic_load,
atomic_readandclear, atomic_set, atomic_subtract, atomic_store -- atomic
operations
#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_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);
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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_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 i386 architecture.
The atomic_cmpset() function returns the result of the compare operation.
The atomic_load() and atomic_readandclear() functions return the value at
the specified address.
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.
#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)
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.
FreeBSD 5.2.1 October 27, 2000 FreeBSD 5.2.1 [ Back ] |
