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24.\" $FreeBSD$ 25.\" 26.Dd September 27, 2005 27.Os 28.Dt ATOMIC 9 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.rm LB RB La Ra 66.Sh DESCRIPTION 67Each of the atomic operations is guaranteed to be atomic in the presence of 68interrupts. 69They can be used to implement reference counts or as building blocks for more 70advanced synchronization primitives such as mutexes. 71.Ss Types 72Each atomic operation operates on a specific 73.Fa type . 74The type to use is indicated in the function name. 75The available types that can be used are: 76.Pp 77.Bl -tag -offset indent -width short -compact 78.It Li int 79unsigned integer 80.It Li long 81unsigned long integer 82.It Li ptr 83unsigned integer the size of a pointer 84.It Li 32 85unsigned 32-bit integer 86.It Li 64 87unsigned 64-bit integer 88.El 89.Pp 90For example, the function to atomically add two integers is called 91.Fn atomic_add_int . 92.Pp 93Certain architectures also provide operations for types smaller than 94.Dq Li int . 95.Pp 96.Bl -tag -offset indent -width short -compact 97.It Li char 98unsigned character 99.It Li short 100unsigned short integer 101.It Li 8 102unsigned 8-bit integer 103.It Li 16 104unsigned 16-bit integer 105.El 106.Pp 107These must not be used in MI code because the instructions to implement them 108efficiently may not be available. 109.Ss Memory Barriers 110Memory barriers are used to guarantee the order of data accesses in 111two ways. 112First, they specify hints to the compiler to not re-order or optimize the 113operations. 114Second, on architectures that do not guarantee ordered data accesses, 115special instructions or special variants of instructions are used to indicate 116to the processor that data accesses need to occur in a certain order. 117As a result, most of the atomic operations have three variants in order to 118include optional memory barriers. 119The first form just performs the operation without any explicit barriers. 120The second form uses a read memory barrier, and the third variant uses a write 121memory barrier. 122.Pp 123The second variant of each operation includes a read memory barrier. 124This barrier ensures that the effects of this operation are completed before the 125effects of any later data accesses. 126As a result, the operation is said to have acquire semantics as it acquires a 127pseudo-lock requiring further operations to wait until it has completed. 128To denote this, the suffix 129.Dq Li _acq 130is inserted into the function name immediately prior to the 131.Dq Li _ Ns Aq Fa type 132suffix. 133For example, to subtract two integers ensuring that any later writes will 134happen after the subtraction is performed, use 135.Fn atomic_subtract_acq_int . 136.Pp 137The third variant of each operation includes a write memory barrier. 138This ensures that all effects of all previous data accesses are completed 139before this operation takes place. 140As a result, the operation is said to have release semantics as it releases 141any pending data accesses to be completed before its operation is performed. 142To denote this, the suffix 143.Dq Li _rel 144is inserted into the function name immediately prior to the 145.Dq Li _ Ns Aq Fa type 146suffix. 147For example, to add two long integers ensuring that all previous 148writes will happen first, use 149.Fn atomic_add_rel_long . 150.Pp 151A practical example of using memory barriers is to ensure that data accesses 152that are protected by a lock are all performed while the lock is held. 153To achieve this, one would use a read barrier when acquiring the lock to 154guarantee that the lock is held before any protected operations are performed. 155Finally, one would use a write barrier when releasing the lock to ensure that 156all of the protected operations are completed before the lock is released. 157.Ss Multiple Processors 158The current set of atomic operations do not necessarily guarantee atomicity 159across multiple processors. 160To guarantee atomicity across processors, not only does the individual 161operation need to be atomic on the processor performing the operation, but 162the result of the operation needs to be pushed out to stable storage and the 163caches of all other processors on the system need to invalidate any cache 164lines that include the affected memory region. 165On the 166.Tn i386 167architecture, the cache coherency model requires that the hardware perform 168this task, thus the atomic operations are atomic across multiple processors. 169On the 170.Tn ia64 171architecture, coherency is only guaranteed for pages that are configured to 172using a caching policy of either uncached or write back. 173.Ss Semantics 174This section describes the semantics of each operation using a C like notation. 175.Bl -hang 176.It Fn atomic_add p v 177.Bd -literal -compact 178*p += v; 179.Ed 180.It Fn atomic_clear p v 181.Bd -literal -compact 182*p &= ~v; 183.Ed 184.It Fn atomic_cmpset dst old new 185.Bd -literal -compact 186if (*dst == old) { 187 *dst = new; 188 return 1; 189} else 190 return 0; 191.Ed 192.El 193.Pp 194The 195.Fn atomic_cmpset 196functions are not implemented for the types 197.Dq Li char , 198.Dq Li short , 199.Dq Li 8 , 200and 201.Dq Li 16 . 202.Bl -hang 203.It Fn atomic_fetchadd p v 204.Bd -literal -compact 205tmp = *p; 206*p += v; 207return tmp; 208.Ed 209.El 210.Pp 211The 212.Fn atomic_fetchadd 213functions are only implemented for the types 214.Dq Li int 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 addr 220.Bd -literal -compact 221return (*addr) 222.Ed 223.El 224.Pp 225The 226.Fn atomic_load 227functions always have acquire semantics. 228.Bl -hang 229.It Fn atomic_readandclear addr 230.Bd -literal -compact 231temp = *addr; 232*addr = 0; 233return (temp); 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 247not have any variants with memory barriers at this time. 248.Bl -hang 249.It Fn atomic_set p v 250.Bd -literal -compact 251*p |= v; 252.Ed 253.It Fn atomic_subtract p v 254.Bd -literal -compact 255*p -= v; 256.Ed 257.It Fn atomic_store p v 258.Bd -literal -compact 259*p = v; 260.Ed 261.El 262.Pp 263The 264.Fn atomic_store 265functions always have release semantics. 266.Pp 267The type 268.Dq Li 64 269is currently not implemented for any of the atomic operations on the 270.Tn arm , 271.Tn i386 , 272and 273.Tn powerpc 274architectures. 275.Sh RETURN VALUES 276The 277.Fn atomic_cmpset 278function 279returns the result of the compare operation. 280The 281.Fn atomic_fetchadd , 282.Fn atomic_load , 283and 284.Fn atomic_readandclear 285functions 286return the value at the specified address. 287.Sh EXAMPLES 288This example uses the 289.Fn atomic_cmpset_acq_ptr 290and 291.Fn atomic_set_ptr 292functions to obtain a sleep mutex and handle recursion. 293Since the 294.Va mtx_lock 295member of a 296.Vt "struct mtx" 297is a pointer, the 298.Dq Li ptr 299type is used. 300.Bd -literal 301/* Try to obtain mtx_lock once. */ 302#define _obtain_lock(mp, tid) \\ 303 atomic_cmpset_acq_ptr(&(mp)->mtx_lock, MTX_UNOWNED, (tid)) 304 305/* Get a sleep lock, deal with recursion inline. */ 306#define _get_sleep_lock(mp, tid, opts, file, line) do { \\ 307 uintptr_t _tid = (uintptr_t)(tid); \\ 308 \\ 309 if (!_obtain_lock(mp, tid)) { \\ 310 if (((mp)->mtx_lock & MTX_FLAGMASK) != _tid) \\ 311 _mtx_lock_sleep((mp), _tid, (opts), (file), (line));\\ 312 else { \\ 313 atomic_set_ptr(&(mp)->mtx_lock, MTX_RECURSE); \\ 314 (mp)->mtx_recurse++; \\ 315 } \\ 316 } \\ 317} while (0) 318.Ed 319.Sh HISTORY 320The 321.Fn atomic_add , 322.Fn atomic_clear , 323.Fn atomic_set , 324and 325.Fn atomic_subtract 326operations were first introduced in 327.Fx 3.0 . 328This first set only supported the types 329.Dq Li char , 330.Dq Li short , 331.Dq Li int , 332and 333.Dq Li long . 334The 335.Fn atomic_cmpset , 336.Fn atomic_load , 337.Fn atomic_readandclear , 338and 339.Fn atomic_store 340operations were added in 341.Fx 5.0 . 342The types 343.Dq Li 8 , 344.Dq Li 16 , 345.Dq Li 32 , 346.Dq Li 64 , 347and 348.Dq Li ptr 349and all of the acquire and release variants 350were added in 351.Fx 5.0 352as well. 353The 354.Fn atomic_fetchadd 355operations were added in 356.Fx 6.0 . 357