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ATOMIC(9)              FreeBSD Kernel Developer's Manual             ATOMIC(9)

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_testandclear_<type>(volatile _type_ *p, u_int v);

     int
     atomic_testandset_<type>(volatile _type_ *p, u_int v);

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.

   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 might not be available.

   Acquire and Release Operations
     By default, a thread's accesses to different memory locations might not
     be performed in 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 FreeBSD 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 acquire and release semantics.

     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.

     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 ``_acq''
     is inserted into the function name immediately prior to the ``_<type>''
     suffix.  For example, to subtract two integers ensuring that subsequent
     loads and stores happen after the subtraction is performed, use
     atomic_subtract_acq_int().

     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 ``_rel'' is inserted into the function name immediately prior to
     the ``_<type>'' suffix.  For example, to add two long integers ensuring
     that all prior loads and stores happen before the addition, use
     atomic_add_rel_long().

     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.

   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,
     VM_MEMATTR_DEFAULT, is guaranteed by all architectures that are supported
     by FreeBSD.  For example, cache coherence is guaranteed on write-back
     memory by the amd64 and 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.

   Semantics
     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_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)
             return (*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)
             *p |= v;

     atomic_subtract(p, v)
             *p -= v;

     atomic_store(p, v)
             *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_testandclear(p, v)
             bit = 1 << (v % (sizeof(*p) * NBBY));
             tmp = (*p & bit) != 0;
             *p &= ~bit;
             return (tmp);

     atomic_testandset(p, v)
             bit = 1 << (v % (sizeof(*p) * NBBY));
             tmp = (*p & bit) != 0;
             *p |= bit;
             return (tmp);

     The atomic_testandset() and atomic_testandclear() 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() and atomic_testandclear() 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.  atomic_testandclear() operation
     was added in FreeBSD 11.0.

FreeBSD 11.0-PRERELEASE          May 12, 2016          FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | HISTORY

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