.\" Copyright (c) 2000-2001 John H. Baldwin .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .Dd October 27, 2000 .Os .Dt ATOMIC 9 .Sh NAME .Nm atomic_add , .Nm atomic_clear , .Nm atomic_cmpset , .Nm atomic_load , .Nm atomic_readandclear , .Nm atomic_set , .Nm atomic_subtract , .Nm atomic_store .Nd atomic operations .Sh SYNOPSIS .In sys/types.h .In machine/atomic.h .Ft void .Fn atomic_add_{acq_,rel_,} "volatile *p" " v" .Ft void .Fn atomic_clear_{acq_,rel_,} "volatile *p" " v" .Ft int .Fo atomic_cmpset_{acq_,rel_,} .Fa "volatile *dst" .Fa " old" .Fa " new" .Fc .Ft .Fn atomic_load_acq_ "volatile *p" .Ft .Fn atomic_readandclear_ "volatile *p" .Ft void .Fn atomic_set_{acq_,rel_,} "volatile *p" " v" .Ft void .Fn atomic_subtract_{acq_,rel_,} "volatile *p" " v" .Ft void .Fn atomic_store_rel_ "volatile *p" " v" .Sh DESCRIPTION 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. .Ss 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: .Bl -tag -offset indent -width short .It char unsigned character .It short unsigned short integer .It int unsigned integer .It long unsigned long integer .It ptr unsigned integer the size of a pointer .It 8 unsigned 8-bit integer .It 16 unsigned 16-bit integer .It 32 unsigned 32-bit integer .It 64 unsigned 64-bit integer .El .Pp For example, the function to atomically add two integers is called .Fn atomic_add_int . .Ss Memory Barriers Memory barriers are used to guarantee the order the order of data accesses in two ways. First, they specify hints to the compiler to not re-order or optimize the operations. Secondly, 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 final variant uses a write memory barrier. .Pp 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 .Dq _acq is inserted into the function name immediately prior to the .Em _type suffix. For example, to subtract two integers ensuring that any later writes will happen after the subtraction is performed, use .Fn atomic_subtract_acq_int . .Pp 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 .Dq _rel is inserted into the function name immediately prior to the .Em _type suffix. For example, to add two long integers ensuring that all previous writes will happen first, use .Fn atomic_add_rel_long . .Pp 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. .Pp .Ss 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 .Tn i386 architecture, the cache coherency model requires that the hardware perform this task, thus the atomic operations are atomic across multiple processors. On the .Tn ia64 architecture, coherency is only guaranteed for pages that are configured to using a caching policy of either uncached or write back. .Ss Semantics This section describes the semantics of each operation using a C like notation. .Bl -hang .It Fn atomic_add "p" "v" .Bd -literal *p += v; .Ed .It Fn atomic_clear "p" "v" .Bd -literal *p &= ~v; .Ed .It Fn atomic_cmpset "dst" "old" "new" .Bd -literal if (*dst == old) { *dst = new; return 1; } else return 0; .Ed .El .Pp The .Fn atomic_cmpset functions are not implemented for the types char, short, 8, and 16. .Bl -hang .It Fn atomic_load "addr" .Bd -literal return (*addr) .Ed .El .Pp The .Fn atomic_load functions always have acquire semantics. .Bl -hang .It Fn atomic_readandclear "addr" .Bd -literal temp = *addr; *addr = 0; return (temp); .Ed .El .Pp The .Fn 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. .Bl -hang .It Fn atomic_set "p" "v" .Bd -literal *p |= v; .Ed .It Fn atomic_subtract "p" "v" .Bd -literal *p -= v; .Ed .It Fn atomic_store "p" "v" .Bd -literal *p = v; .Ed .El .Pp The .Fn atomic_store functions always have release semantics. .Pp The type .Dq 64 is currently not implemented for any of the atomic operations on the .Tn i386 architecture. .Sh RETURN VALUES .Fn atomic_cmpset returns the result of the compare operation. .Fn atomic_load and .Fn atomic_readandclear return the value at the specified address. .Sh EXAMPLES This example uses the .Fn atomic_cmpset_acq_ptr and .Fn atomic_set_ptr functions to obtain a sleep mutex and handle recursion. Since the .Va mtx_lock member of a .Li struct mtx is a pointer, the .Dq ptr type is used. .Bd -literal #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) .Ed .Sh HISTORY The .Fn atomic_add , .Fn atomic_clear , .Fn atomic_set , and .Fn atomic_subtract operations were first introduced in .Fx 3.0 . This first set only suppored the types char, short, int, and long. The .Fn atomic_cmpset , .Fn atomic_load , .Fn atomic_readandclear , and .Fn atomic_store operations were added in .Fx 5.0 . The types 8, 16, 32, 64, and ptr and all of the acquire and release variants were added in .Fx 5.0 as well.