Provided by: freebsd-manpages_11.1-3_all bug

NAME

     atomic_add, atomic_clear, atomic_cmpset, atomic_fcmpset, 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);

     int
     atomic_fcmpset_[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_fcmpset(dst, *old, new)

     On architectures implementing Compare And Swap operation in hardware, the functionality can
     be described as
           if (*dst == *old) {
                   *dst = new;
                   return (1);
           } else {
                   *old = *dst;
                   return (0);
           }
     On architectures which provide Load Linked/Store Conditional primitive, the write to *dst
     might also fail for several reasons, most important of which is a parallel write to *dst
     cache line by other CPU.  In this case atomic_fcmpset() function also returns false, despite
           *old == *dst.

     The atomic_fcmpset() 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_fcmpset() function returns true if the operation succeeded.  Otherwise it returns
     false and sets *old to the found value.  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.