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

NAME
     atomic_add, atomic_clear, atomic_cmpset, atomic_fcmpset, atomic_fetchadd,
     atomic_load, atomic_readandclear, atomic_set, atomic_subtract,
     atomic_store, atomic_thread_fence -- 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);

     void
     atomic_thread_fence_[acq|acq_rel|rel|seq_cst](void);

DESCRIPTION
     Atomic operations are commonly used to implement reference	counts and as
     building blocks for synchronization primitives, such as mutexes.

     All of these operations are performed atomically across multiple threads
     and in the	presence of interrupts,	meaning	that they are performed	in an
     indivisible manner	from the perspective of	concurrently running threads
     and interrupt handlers.

     On	all architectures supported by FreeBSD,	ordinary loads and stores of
     integers in cache-coherent	memory are inherently atomic if	the integer is
     naturally aligned and its size does not exceed the	processor's word size.
     However, such loads and stores may	be elided from the program by the com-
     piler, whereas atomic operations are always performed.

     When atomic operations are	performed on cache-coherent memory, all	opera-
     tions on the same location	are totally ordered.

     When an atomic load is performed on a location in cache-coherent memory,
     it	reads the entire value that was	defined	by the last atomic store to
     each byte of the location.	 An atomic load	will never return a value out
     of	thin air.  When	an atomic store	is performed on	a location, no other
     thread or interrupt handler will observe a	torn write, or partial modifi-
     cation of the location.

     Except as noted below, the	semantics of these operations are almost iden-
     tical to the semantics of similarly named C11 atomic operations.

   Types
     Most atomic operations act	upon a specific	type.  That type is indicated
     in	the function name.  In contrast	to C11 atomic operations, FreeBSD's
     atomic operations are performed on	ordinary integer types.	 The available
     types 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 types must not be used in machine-independent code.

   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 sin-
     gle-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 syn-
     chronization 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, a
     programmer	can use	atomic operations with acquire and release semantics.

     Atomic operations on memory have up to three variants.  The first,	or
     relaxed variant, performs the operation without imposing any ordering
     constraints on accesses to	other memory locations.	 This variant is the
     default.  The second variant has acquire semantics, and the third variant
     has release semantics.

     When an atomic operation has acquire semantics, the operation must	have
     completed before any subsequent load or store (by program order) is per-
     formed.  Conversely, acquire semantics do not require that	prior loads or
     stores have completed before the atomic operation is performed.  An
     atomic operation can only have acquire semantics if it performs a load
     from memory.  To denote acquire semantics,	the suffix ``_acq'' is
     inserted into the function	name immediately prior to the ``_<type>'' suf-
     fix.  For example,	to subtract two	integers ensuring that the subtraction
     is	completed before any subsequent	loads and stores are performed,	use
     atomic_subtract_acq_int().

     When an atomic operation has release semantics, all prior loads or	stores
     (by program order)	must have completed before the operation is performed.
     Conversely, release semantics do not require that the atomic operation
     must have completed before	any subsequent load or store is	performed.  An
     atomic operation can only have release semantics if it performs a store
     to	memory.	 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 are	completed before the addition is performed, use
     atomic_add_rel_long().

     When a release operation by one thread synchronizes with an acquire oper-
     ation by another thread, usually meaning that the acquire operation reads
     the value written by the release operation, then the effects of all prior
     stores by the releasing thread must become	visible	to subsequent loads by
     the acquiring thread.  Moreover, the effects of all stores	(by other
     threads) that were	visible	to the releasing thread	must also become visi-
     ble to the	acquiring thread.  These rules only apply to the synchronizing
     threads.  Other threads might observe these stores	in a different order.

     In	effect,	atomic operations with acquire and release semantics establish
     one-way barriers to reordering that enable	the implementations of syn-
     chronization 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.

   Thread Fence	Operations
     Alternatively, a programmer can use atomic	thread fence operations	to
     constrain the reordering of accesses.  In contrast	to other atomic	opera-
     tions, fences do not, themselves, access memory.

     When a fence has acquire semantics, all prior loads (by program order)
     must have completed before	any subsequent load or store is	performed.
     Thus, an acquire fence is a two-way barrier for load operations.  To
     denote acquire semantics, the suffix ``_acq'' is appended to the function
     name, for example,	atomic_thread_fence_acq().

     When a fence has release semantics, all prior loads or stores (by program
     order) must have completed	before any subsequent store operation is per-
     formed.  Thus, a release fence is a two-way barrier for store operations.
     To	denote release semantics, the suffix ``_rel'' is appended to the func-
     tion name,	for example, atomic_thread_fence_rel().

     Although atomic_thread_fence_acq_rel() implements both acquire and
     release semantics,	it is not a full barrier.  For example,	a store	prior
     to	the fence (in program order) may be completed after a load subsequent
     to	the fence.  In contrast, atomic_thread_fence_seq_cst() implements a
     full barrier.  Neither loads nor stores may cross this barrier in either
     direction.

     In	C11, a release fence by	one thread synchronizes	with an	acquire	fence
     by	another	thread when an atomic load that	is prior to the	acquire	fence
     (by program order)	reads the value	written	by an atomic store that	is
     subsequent	to the release fence.  In constrast, in	FreeBSD, because of
     the atomicity of ordinary,	naturally aligned loads	and stores, fences can
     also be synchronized by ordinary loads and	stores.	 This simplifies the
     implementation and	use of some synchronization primitives in FreeBSD.

     Since neither a compiler nor a processor can foresee which	(atomic) load
     will read the value written by an (atomic)	store, the ordering con-
     straints imposed by fences	must be	more restrictive than acquire loads
     and release stores.  Essentially, this is why fences are two-way barri-
     ers.

     Although fences impose more restrictive ordering than acquire loads and
     release stores, by	separating access from ordering, they can sometimes
     facilitate	more efficient implementations of synchronization primitives.
     For example, they can be used to avoid executing a	memory barrier until a
     memory access shows that some condition is	satisfied.

   Multiple Processors
     In	multiprocessor systems,	the atomicity of the atomic operations on mem-
     ory depends on support for	cache coherence	in the underlying architec-
     ture.  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 architec-
     tures, 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);

     Some architectures	do not implement the atomic_cmpset() functions 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.

     Some architectures	do not implement the atomic_fcmpset() functions	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);

     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 vari-
     ants with memory barriers at this time.

     atomic_set(p, v)
	     *p	|= v;

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

     atomic_store(p, v)
	     *p	= v;

     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	opera-
     tions 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()	func-
     tions 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 introduced	in FreeBSD 3.0.	 Initially, these operations
     were defined on the types ``char'', ``short'', ``int'', and ``long''.

     The atomic_cmpset(), atomic_load_acq(), atomic_readandclear(), and
     atomic_store_rel()	operations were	added in FreeBSD 5.0.  Simultaneously,
     the acquire and release variants were introduced, and support was added
     for operation on the types	``8'', ``16'', ``32'', ``64'', and ``ptr''.

     The atomic_fetchadd() operation was added in FreeBSD 6.0.

     The atomic_swap() and atomic_testandset() operations were added in
     FreeBSD 10.0.

     The atomic_testandclear() and atomic_thread_fence() operations were added
     in	FreeBSD	11.0.

     The relaxed variants of atomic_load() and atomic_store() were added in
     FreeBSD 12.0.

FreeBSD	Ports 11.2	       December	22, 2017	    FreeBSD Ports 11.2

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

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