1.\" Copyright (c) 2000-2001 John H. Baldwin <jhb@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 14.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 15.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 16.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 17.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 18.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 19.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 20.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 21.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 22.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 23.\" 24.\" $FreeBSD$ 25.\" 26.Dd August 20, 2013 27.Dt ATOMIC 9 28.Os 29.Sh NAME 30.Nm atomic_add , 31.Nm atomic_clear , 32.Nm atomic_cmpset , 33.Nm atomic_fetchadd , 34.Nm atomic_load , 35.Nm atomic_readandclear , 36.Nm atomic_set , 37.Nm atomic_subtract , 38.Nm atomic_store 39.Nd atomic operations 40.Sh SYNOPSIS 41.In sys/types.h 42.In machine/atomic.h 43.Ft void 44.Fn atomic_add_[acq_|rel_]<type> "volatile <type> *p" "<type> v" 45.Ft void 46.Fn atomic_clear_[acq_|rel_]<type> "volatile <type> *p" "<type> v" 47.Ft int 48.Fo atomic_cmpset_[acq_|rel_]<type> 49.Fa "volatile <type> *dst" 50.Fa "<type> old" 51.Fa "<type> new" 52.Fc 53.Ft <type> 54.Fn atomic_fetchadd_<type> "volatile <type> *p" "<type> v" 55.Ft <type> 56.Fn atomic_load_acq_<type> "volatile <type> *p" 57.Ft <type> 58.Fn atomic_readandclear_<type> "volatile <type> *p" 59.Ft void 60.Fn atomic_set_[acq_|rel_]<type> "volatile <type> *p" "<type> v" 61.Ft void 62.Fn atomic_subtract_[acq_|rel_]<type> "volatile <type> *p" "<type> v" 63.Ft void 64.Fn atomic_store_rel_<type> "volatile <type> *p" "<type> v" 65.Ft <type> 66.Fn atomic_swap_<type> "volatile <type> *p" "<type> v" 67.Ft int 68.Fn atomic_testandset_<type> "volatile <type> *p" "u_int v" 69.Sh DESCRIPTION 70Each of the atomic operations is guaranteed to be atomic in the presence of 71interrupts. 72They can be used to implement reference counts or as building blocks for more 73advanced synchronization primitives such as mutexes. 74.Ss Types 75Each atomic operation operates on a specific 76.Fa type . 77The type to use is indicated in the function name. 78The available types that can be used are: 79.Pp 80.Bl -tag -offset indent -width short -compact 81.It Li int 82unsigned integer 83.It Li long 84unsigned long integer 85.It Li ptr 86unsigned integer the size of a pointer 87.It Li 32 88unsigned 32-bit integer 89.It Li 64 90unsigned 64-bit integer 91.El 92.Pp 93For example, the function to atomically add two integers is called 94.Fn atomic_add_int . 95.Pp 96Certain architectures also provide operations for types smaller than 97.Dq Li int . 98.Pp 99.Bl -tag -offset indent -width short -compact 100.It Li char 101unsigned character 102.It Li short 103unsigned short integer 104.It Li 8 105unsigned 8-bit integer 106.It Li 16 107unsigned 16-bit integer 108.El 109.Pp 110These must not be used in MI code because the instructions to implement them 111efficiently may not be available. 112.Ss Memory Barriers 113Memory barriers are used to guarantee the order of data accesses in 114two ways. 115First, they specify hints to the compiler to not re-order or optimize the 116operations. 117Second, on architectures that do not guarantee ordered data accesses, 118special instructions or special variants of instructions are used to indicate 119to the processor that data accesses need to occur in a certain order. 120As a result, most of the atomic operations have three variants in order to 121include optional memory barriers. 122The first form just performs the operation without any explicit barriers. 123The second form uses a read memory barrier, and the third variant uses a write 124memory barrier. 125.Pp 126The second variant of each operation includes a read memory barrier. 127This barrier ensures that the effects of this operation are completed before the 128effects of any later data accesses. 129As a result, the operation is said to have acquire semantics as it acquires a 130pseudo-lock requiring further operations to wait until it has completed. 131To denote this, the suffix 132.Dq Li _acq 133is inserted into the function name immediately prior to the 134.Dq Li _ Ns Aq Fa type 135suffix. 136For example, to subtract two integers ensuring that any later writes will 137happen after the subtraction is performed, use 138.Fn atomic_subtract_acq_int . 139.Pp 140The third variant of each operation includes a write memory barrier. 141This ensures that all effects of all previous data accesses are completed 142before this operation takes place. 143As a result, the operation is said to have release semantics as it releases 144any pending data accesses to be completed before its operation is performed. 145To denote this, the suffix 146.Dq Li _rel 147is inserted into the function name immediately prior to the 148.Dq Li _ Ns Aq Fa type 149suffix. 150For example, to add two long integers ensuring that all previous 151writes will happen first, use 152.Fn atomic_add_rel_long . 153.Pp 154A practical example of using memory barriers is to ensure that data accesses 155that are protected by a lock are all performed while the lock is held. 156To achieve this, one would use a read barrier when acquiring the lock to 157guarantee that the lock is held before any protected operations are performed. 158Finally, one would use a write barrier when releasing the lock to ensure that 159all of the protected operations are completed before the lock is released. 160.Ss Multiple Processors 161The current set of atomic operations do not necessarily guarantee atomicity 162across multiple processors. 163To guarantee atomicity across processors, not only does the individual 164operation need to be atomic on the processor performing the operation, but 165the result of the operation needs to be pushed out to stable storage and the 166caches of all other processors on the system need to invalidate any cache 167lines that include the affected memory region. 168On the 169.Tn i386 170architecture, the cache coherency model requires that the hardware perform 171this task, thus the atomic operations are atomic across multiple processors. 172.Ss Semantics 173This section describes the semantics of each operation using a C like notation. 174.Bl -hang 175.It Fn atomic_add p v 176.Bd -literal -compact 177*p += v; 178.Ed 179.It Fn atomic_clear p v 180.Bd -literal -compact 181*p &= ~v; 182.Ed 183.It Fn atomic_cmpset dst old new 184.Bd -literal -compact 185if (*dst == old) { 186 *dst = new; 187 return (1); 188} else 189 return (0); 190.Ed 191.El 192.Pp 193The 194.Fn atomic_cmpset 195functions are not implemented for the types 196.Dq Li char , 197.Dq Li short , 198.Dq Li 8 , 199and 200.Dq Li 16 . 201.Bl -hang 202.It Fn atomic_fetchadd p v 203.Bd -literal -compact 204tmp = *p; 205*p += v; 206return (tmp); 207.Ed 208.El 209.Pp 210The 211.Fn atomic_fetchadd 212functions are only implemented for the types 213.Dq Li int , 214.Dq Li long 215and 216.Dq Li 32 217and do not have any variants with memory barriers at this time. 218.Bl -hang 219.It Fn atomic_load p 220.Bd -literal -compact 221return (*p); 222.Ed 223.El 224.Pp 225The 226.Fn atomic_load 227functions are only provided with acquire memory barriers. 228.Bl -hang 229.It Fn atomic_readandclear p 230.Bd -literal -compact 231tmp = *p; 232*p = 0; 233return (tmp); 234.Ed 235.El 236.Pp 237The 238.Fn atomic_readandclear 239functions are not implemented for the types 240.Dq Li char , 241.Dq Li short , 242.Dq Li ptr , 243.Dq Li 8 , 244and 245.Dq Li 16 246and do not have any variants with memory barriers at this time. 247.Bl -hang 248.It Fn atomic_set p v 249.Bd -literal -compact 250*p |= v; 251.Ed 252.It Fn atomic_subtract p v 253.Bd -literal -compact 254*p -= v; 255.Ed 256.It Fn atomic_store p v 257.Bd -literal -compact 258*p = v; 259.Ed 260.El 261.Pp 262The 263.Fn atomic_store 264functions are only provided with release memory barriers. 265.Bl -hang 266.It Fn atomic_swap p v 267.Bd -literal -compact 268tmp = *p; 269*p = v; 270return (tmp); 271.Ed 272.El 273.Pp 274The 275.Fn atomic_swap 276functions are not implemented for the types 277.Dq Li char , 278.Dq Li short , 279.Dq Li ptr , 280.Dq Li 8 , 281and 282.Dq Li 16 283and do not have any variants with memory barriers at this time. 284.Bl -hang 285.It Fn atomic_testandset p v 286.Bd -literal -compact 287bit = 1 << (v % (sizeof(*p) * NBBY)); 288tmp = (*p & bit) != 0; 289*p |= bit; 290return (tmp); 291.Ed 292.El 293.Pp 294The 295.Fn atomic_testandset 296functions are only implemented for the types 297.Dq Li int , 298.Dq Li long 299and 300.Dq Li 32 301and do not have any variants with memory barriers at this time. 302.Pp 303The type 304.Dq Li 64 305is currently not implemented for any of the atomic operations on the 306.Tn arm , 307.Tn i386 , 308and 309.Tn powerpc 310architectures. 311.Sh RETURN VALUES 312The 313.Fn atomic_cmpset 314function returns the result of the compare operation. 315The 316.Fn atomic_fetchadd , 317.Fn atomic_load , 318.Fn atomic_readandclear , 319and 320.Fn atomic_swap 321functions return the value at the specified address. 322The 323.Fn atomic_testandset 324function returns the result of the test operation. 325.Sh EXAMPLES 326This example uses the 327.Fn atomic_cmpset_acq_ptr 328and 329.Fn atomic_set_ptr 330functions to obtain a sleep mutex and handle recursion. 331Since the 332.Va mtx_lock 333member of a 334.Vt "struct mtx" 335is a pointer, the 336.Dq Li ptr 337type is used. 338.Bd -literal 339/* Try to obtain mtx_lock once. */ 340#define _obtain_lock(mp, tid) \\ 341 atomic_cmpset_acq_ptr(&(mp)->mtx_lock, MTX_UNOWNED, (tid)) 342 343/* Get a sleep lock, deal with recursion inline. */ 344#define _get_sleep_lock(mp, tid, opts, file, line) do { \\ 345 uintptr_t _tid = (uintptr_t)(tid); \\ 346 \\ 347 if (!_obtain_lock(mp, tid)) { \\ 348 if (((mp)->mtx_lock & MTX_FLAGMASK) != _tid) \\ 349 _mtx_lock_sleep((mp), _tid, (opts), (file), (line));\\ 350 else { \\ 351 atomic_set_ptr(&(mp)->mtx_lock, MTX_RECURSE); \\ 352 (mp)->mtx_recurse++; \\ 353 } \\ 354 } \\ 355} while (0) 356.Ed 357.Sh HISTORY 358The 359.Fn atomic_add , 360.Fn atomic_clear , 361.Fn atomic_set , 362and 363.Fn atomic_subtract 364operations were first introduced in 365.Fx 3.0 . 366This first set only supported the types 367.Dq Li char , 368.Dq Li short , 369.Dq Li int , 370and 371.Dq Li long . 372The 373.Fn atomic_cmpset , 374.Fn atomic_load , 375.Fn atomic_readandclear , 376and 377.Fn atomic_store 378operations were added in 379.Fx 5.0 . 380The types 381.Dq Li 8 , 382.Dq Li 16 , 383.Dq Li 32 , 384.Dq Li 64 , 385and 386.Dq Li ptr 387and all of the acquire and release variants 388were added in 389.Fx 5.0 390as well. 391The 392.Fn atomic_fetchadd 393operations were added in 394.Fx 6.0 . 395The 396.Fn atomic_swap 397and 398.Fn atomic_testandset 399operations were added in 400.Fx 10.0 . 401