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     atomic_add, atomic_clear, atomic_cmpset, atomic_fetchadd, atomic_load,
     atomic_readandclear, atomic_set, atomic_subtract, atomic_store — atomic


     #include <sys/types.h>
     #include <machine/atomic.h>

     atomic_add_[acq_|rel_]<type>(volatile <type> *p, <type> v);

     atomic_clear_[acq_|rel_]<type>(volatile <type> *p, <type> v);

     atomic_cmpset_[acq_|rel_]<type>(volatile <type> *dst, <type> old,
         <type> new);

     atomic_fetchadd_<type>(volatile <type> *p, <type> v);

     atomic_load_acq_<type>(volatile <type> *p);

     atomic_readandclear_<type>(volatile <type> *p);

     atomic_set_[acq_|rel_]<type>(volatile <type> *p, <type> v);

     atomic_subtract_[acq_|rel_]<type>(volatile <type> *p, <type> v);

     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

     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

           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

     Certain architectures also provide operations for types smaller than

           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.

     This section describes the semantics of each operation using a C like

     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”,
     “long” and “32” and do not have any variants with memory barriers at this

             return (*addr)

     The atomic_load() functions are only provided with acquire memory

             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 are only provided with release memory

     The type “64” is currently not implemented for any of the atomic
     operations on the arm, i386, and powerpc architectures.


     The atomic_cmpset() function returns the result of the compare operation.
     The atomic_fetchadd(), 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.

     /* 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)


     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.