.\" $NetBSD: atomic_loadstore.9,v 1.7 2022/04/09 23:32:53 riastradh Exp $ .\" .\" Copyright (c) 2019 The NetBSD Foundation .\" All rights reserved. .\" .\" This code is derived from software contributed to The NetBSD Foundation .\" by Taylor R. Campbell. .\" .\" 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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``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 FOUNDATION OR CONTRIBUTORS .\" 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. .\" .Dd February 11, 2022 .Dt ATOMIC_LOADSTORE 9 .Os .Sh NAME .Nm atomic_load_relaxed , .Nm atomic_load_acquire , .Nm atomic_load_consume , .Nm atomic_store_relaxed , .Nm atomic_store_release .Nd atomic and ordered memory operations .Sh SYNOPSIS .In sys/atomic.h .Ft T .Fn atomic_load_relaxed "const volatile T *p" .Ft T .Fn atomic_load_acquire "const volatile T *p" .Ft T .Fn atomic_load_consume "const volatile T *p" .Ft void .Fn atomic_store_relaxed "volatile T *p" "T v" .Ft void .Fn atomic_store_release "volatile T *p" "T v" .Sh DESCRIPTION These type-generic macros implement memory operations that are .Em atomic and that have .Em memory ordering constraints . Aside from atomicity and ordering, the load operations are equivalent to .Li * Ns Fa p and the store operations are equivalent to .Li * Ns Fa p Li "=" Fa v . The pointer .Fa p must be aligned, even on architectures like x86 which generally lack strict alignment requirements; see .Sx SIZE AND ALIGNMENT for details. .Pp .Em Atomic means that the memory operations cannot be .Em fused or .Em torn : .Bl -bullet .It .Em Fusing is combining multiple memory operations on a single object into one memory operation, such as replacing .Bd -literal -compact *p = v; x = *p; .Ed by .Bd -literal -compact *p = v; x = v; .Ed since the compiler can prove that .Li \&*p will yield .Li v after .Li \&*p\ =\ v . For .Em atomic memory operations, the implementation .Em will not assume that .Bl -dash -compact .It consecutive loads of the same object will return the same value, or .It a store followed by a load of the same object will return the value stored, or .It consecutive stores of the same object are redundant. .El Thus, the implementation will not replace two consecutive atomic loads by one, will not elide an atomic load following a store, and will not combine two consecutive atomic stores into one. .Pp For example, .Bd -literal atomic_store_relaxed(&flag, 1); while (atomic_load_relaxed(&flag)) continue; .Ed .Pp may be used to set a flag and then busy-wait until another thread clears it, whereas .Bd -literal flag = 1; while (flag) continue; .Ed .Pp may be transformed into the infinite loop .Bd -literal flag = 1; while (1) continue; .Ed .It .Em Tearing is implementing a memory operation on a large data unit such as a 32-bit word by issuing multiple memory operations on smaller data units such as 8-bit bytes. The implementation will not tear .Em atomic loads or stores into smaller ones. Thus, as far as any interrupt, other thread, or other CPU can tell, an atomic memory operation is issued either all at once or not at all. .Pp For example, if a 32-bit word .Va w is written with .Pp .Dl atomic_store_relaxed(&w,\ 0x00010002); .Pp then an interrupt, other thread, or other CPU reading it with .Li atomic_load_relaxed(&w) will never witness it partially written, whereas .Pp .Dl w\ =\ 0x00010002; .Pp might be compiled into a pair of separate 16-bit store instructions instead of one single word-sized store instruction, in which case other threads may see the intermediate state with only one of the halves written. .El .Pp Atomic operations on any single object occur in a total order shared by all interrupts, threads, and CPUs, which is consistent with the program order in every interrupt, thread, and CPU. A single program without interruption or other threads or CPUs will always observe its own loads and stores in program order, but another program in an interrupt handler, in another thread, or on another CPU may issue loads that return values as if the first program's stores occurred out of program order, and vice versa. Two different threads might each observe a third thread's memory operations in different orders. .Pp The .Em memory ordering constraints make limited guarantees of ordering relative to memory operations on .Em other objects as witnessed by interrupts, other threads, or other CPUs, and have the following meanings: .Bl -tag -width relaxed .It relaxed No ordering relative to memory operations on any other objects is guaranteed. Relaxed ordering is the default for ordinary non-atomic memory operations like .Li "*p" and .Li "*p = v" . .Pp Atomic operations with relaxed ordering are cheap: they are not read/modify/write atomic operations, and they do not involve any kind of inter-CPU ordering barriers. .It acquire This memory operation happens before all subsequent memory operations in program order. However, prior memory operations in program order may be reordered to happen after this one. For example, assuming no aliasing between the pointers, the implementation is allowed to treat .Bd -literal int x = *p; if (atomic_load_acquire(q)) { int y = *r; *s = x + y; return 1; } .Ed .Pp as if it were .Bd -literal if (atomic_load_acquire(q)) { int x = *p; int y = *r; *s = x + y; return 1; } .Ed .Pp but .Em not as if it were .Bd -literal int x = *p; int y = *r; *s = x + y; if (atomic_load_acquire(q)) { return 1; } .Ed .It consume This memory operation happens before all memory operations on objects at addresses that are computed from the value returned by this one. Otherwise, no ordering relative to memory operations on other objects is implied. .Pp For example, the implementation is allowed to treat .Bd -literal struct foo *foo0, *foo1; struct foo *f0 = atomic_load_consume(&foo0); struct foo *f1 = atomic_load_consume(&foo1); int x = f0->x; int y = f1->y; .Ed .Pp as if it were .Bd -literal struct foo *foo0, *foo1; struct foo *f1 = atomic_load_consume(&foo1); struct foo *f0 = atomic_load_consume(&foo0); int y = f1->y; int x = f0->x; .Ed .Pp but loading .Li f0->x is guaranteed to happen after loading .Li foo0 even if the CPU had a cached value for the address that .Li f0->x happened to be at, and likewise for .Li f1->y and .Li foo1 . .Pp .Fn atomic_load_consume functions like .Fn atomic_load_acquire as long as the memory operations that must happen after it are limited to addresses that depend on the value returned by it, but it is almost always as cheap as .Fn atomic_load_relaxed . See .Sx ACQUIRE OR CONSUME? below for more details. .It release All prior memory operations in program order happen before this one. However, subsequent memory operations in program order may be reordered to happen before this one too. For example, assuming no aliasing between the pointers, the implementation is allowed to treat .Bd -literal int x = *p; *q = x; atomic_store_release(r, 0); int y = *s; return x + y; .Ed .Pp as if it were .Bd -literal int y = *s; int x = *p; *q = x; atomic_store_release(r, 0); return x + y; .Ed .Pp but .Em not as if it were .Bd -literal atomic_store_release(r, 0); int x = *p; int y = *s; *q = x; return x + y; .Ed .El .Ss PAIRING ORDERED MEMORY OPERATIONS In general, each .Fn atomic_store_release .Em must be paired with either .Fn atomic_load_acquire or .Fn atomic_load_consume in order to have an effect \(em it is only when a release operation synchronizes with an acquire or consume operation that any ordering guaranteed between memory operations .Em before the release operation and memory operations .Em after the acquire/consume operation. .Pp For example, to set up an entry in a table and then mark the entry ready, you should: .Bl -enum .It Perform memory operations to initialize the data. .Bd -literal tab[i].x = ...; tab[i].y = ...; .Ed .It Issue .Fn atomic_store_release to mark it ready. .Bd -literal atomic_store_release(&tab[i].ready, 1); .Ed .It Possibly in another thread, issue .Fn atomic_load_acquire to ascertain whether it is ready. .Bd -literal if (atomic_load_acquire(&tab[i].ready) == 0) return EWOULDBLOCK; .Ed .It Perform memory operations to use the data. .Bd -literal do_stuff(tab[i].x, tab[i].y); .Ed .El .Pp Similarly, if you want to create an object, initialize it, and then publish it to be used by another thread, then you should: .Bl -enum .It Perform memory operations to initialize the object. .Bd -literal struct mumble *m = kmem_alloc(sizeof(*m), KM_SLEEP); m->x = x; m->y = y; m->z = m->x + m->y; .Ed .It Issue .Fn atomic_store_release to publish it. .Bd -literal atomic_store_release(&the_mumble, m); .Ed .It Possibly in another thread, issue .Fn atomic_load_consume to get it. .Bd -literal struct mumble *m = atomic_load_consume(&the_mumble); .Ed .It Perform memory operations to use the object's members. .Bd -literal m->y &= m->x; do_things(m->x, m->y, m->z); .Ed .El .Pp In both examples, assuming that the value written by .Fn atomic_store_release in step\~2 is read by .Fn atomic_load_acquire or .Fn atomic_load_consume in step\~3, this guarantees that all of the memory operations in step\~1 complete before any of the memory operations in step\~4 \(em even if they happen on different CPUs. .Pp Without .Em both the release operation in step\~2 .Em and the acquire or consume operation in step\~3, no ordering is guaranteed between the memory operations in steps\~1 and\~4. In fact, without .Em both release and acquire/consume, even the assignment .Li m->z\ =\ m->x\ +\ m->y in step\~1 might read values of .Li m->x and .Li m->y that were written in step\~4. .Ss ACQUIRE OR CONSUME? You must use .Fn atomic_load_acquire when subsequent memory operations in program order that must happen after the load are on objects at .Em addresses that might not depend arithmetically on the resulting value . This applies particularly when the choice of whether to do the subsequent memory operation depends on a .Em control-flow decision based on the resulting value : .Bd -literal struct gadget { int ready, x; } the_gadget; /* Producer */ the_gadget.x = 42; atomic_store_release(&the_gadget.ready, 1); /* Consumer */ if (atomic_load_acquire(&the_gadget.ready) == 0) return EWOULDBLOCK; int x = the_gadget.x; .Ed .Pp Here the .Em decision of whether to load .Li the_gadget.x depends on a control-flow decision depending on value loaded from .Li the_gadget.ready , and loading .Li the_gadget.x must happen after loading .Li the_gadget.ready . Using .Fn atomic_load_acquire guarantees that the compiler and CPU do not conspire to load .Li the_gadget.x before we have ascertained that it is ready. .Pp You may use .Fn atomic_load_consume if all subsequent memory operations in program order that must happen after the load are performed on objects at .Em addresses computed arithmetically from the resulting value , such as loading a pointer to a structure object and then dereferencing it: .Bd -literal struct gizmo { int x, y, z; }; struct gizmo null_gizmo; struct gizmo *the_gizmo = &null_gizmo; /* Producer */ struct gizmo *g = kmem_alloc(sizeof(*g), KM_SLEEP); g->x = 12; g->y = 34; g->z = 56; atomic_store_release(&the_gizmo, g); /* Consumer */ struct gizmo *g = atomic_load_consume(&the_gizmo); int y = g->y; .Ed .Pp Here the .Em address of .Li g->y depends on the value of the pointer loaded from .Li the_gizmo . Using .Fn atomic_load_consume guarantees that we do not witness a stale cache for that address. .Pp In some cases it may be unclear. For example: .Bd -literal int x[2]; bool b; /* Producer */ x[0] = 42; atomic_store_release(&b, 0); /* Consumer 1 */ int y = atomic_load_???(&b) ? x[0] : x[1]; /* Consumer 2 */ int y = x[atomic_load_???(&b) ? 0 : 1]; /* Consumer 3 */ int y = x[atomic_load_???(&b) ^ 1]; .Ed .Pp Although the three consumers seem to be equivalent, by the letter of C11 consumers\~1 and\~2 require .Fn atomic_load_acquire because the value determines the address of a subsequent load only via control-flow decisions in the .Li ?: operator, whereas consumer\~3 can use .Fn atomic_load_consume . However, if you're not sure, you should err on the side of .Fn atomic_load_acquire until C11 implementations have ironed out the kinks in the semantics. .Pp On all CPUs other than DEC Alpha, .Fn atomic_load_consume is cheap \(em it is identical to .Fn atomic_load_relaxed . In contrast, .Fn atomic_load_acquire usually implies an expensive memory barrier. .Ss SIZE AND ALIGNMENT The pointer .Fa p must be aligned \(em that is, if the object it points to is .\" 2\c .ie t \s-2\v'-0.4m'n\v'+0.4m'\s+2 .el ^n .\" bytes long, then the low-order .Ar n bits of .Fa p must be zero. .Pp All .Nx ports support atomic loads and stores on units of data up to 32 bits. Some ports additionally support atomic loads and stores on larger quantities, like 64-bit quantities, if .Dv __HAVE_ATOMIC64_LOADSTORE is defined. The macros are not allowed on larger quantities of data than the port supports atomically; attempts to use them for such quantities should result in a compile-time assertion failure. .Pp For example, as long as you use .Fn atomic_store_* to write a 32-bit quantity, you can safely use .Fn atomic_load_relaxed to optimistically read it outside a lock, but for a 64-bit quantity it must be conditional on .Dv __HAVE_ATOMIC64_LOADSTORE \(em otherwise it will lead to compile-time errors on platforms without 64-bit atomic loads and stores: .Bd -literal struct foo { kmutex_t f_lock; uint32_t f_refcnt; uint64_t f_ticket; }; if (atomic_load_relaxed(&foo->f_refcnt) == 0) return 123; #ifdef __HAVE_ATOMIC64_LOADSTORE if (atomic_load_relaxed(&foo->f_ticket) == ticket) return 123; #endif mutex_enter(&foo->f_lock); if (foo->f_refcnt == 0 || foo->f_ticket == ticket) ret = 123; ... #ifdef __HAVE_ATOMIC64_LOADSTORE atomic_store_relaxed(&foo->f_ticket, foo->f_ticket + 1); #else foo->f_ticket++; #endif ... mutex_exit(&foo->f_lock); .Ed .Sh C11 COMPATIBILITY These macros are meant to follow .Tn C11 semantics, in terms of .Li atomic_load_explicit() and .Li atomic_store_explicit() with the appropriate memory order specifiers, and are meant to make future adoption of the .Tn C11 atomic API easier. Eventually it may be mandatory to use the .Tn C11 .Vt _Atomic type qualifier or equivalent for the operands. .Sh LINUX ANALOGUES The Linux kernel provides two macros .Li READ_ONCE(x) and .Li WRITE_ONCE(x,\ v) which are similar to .Li atomic_load_consume(&x) and .Li atomic_store_relaxed(&x,\ v) , respectively. However, while Linux's .Li READ_ONCE and .Li WRITE_ONCE prevent fusing, they may in some cases be torn \(em and therefore fail to guarantee atomicity \(em because: .Bl -bullet .It They do not require the address .Li "&x" to be aligned. .It They do not require .Li sizeof(x) to be at most the largest size of available atomic loads and stores on the host architecture. .El .Sh MEMORY BARRIERS AND ATOMIC READ/MODIFY/WRITE The atomic read/modify/write operations in .Xr atomic_ops 3 have relaxed ordering by default, but can be combined with the memory barriers in .Xr membar_ops 3 for the same effect as an acquire operation and a release operation for the purposes of pairing with .Fn atomic_store_release and .Fn atomic_load_acquire or .Fn atomic_load_consume . If .Li atomic_r/m/w() is an atomic read/modify/write operation in .Xr atomic_ops 3 , then .Bd -literal membar_release(); atomic_r/m/w(obj, ...); .Ed .Pp functions like a release operation on .Va obj , and .Bd -literal atomic_r/m/w(obj, ...); membar_acquire(); .Ed .Pp functions like a acquire operation on .Va obj . .Pp On architectures where .Dv __HAVE_ATOMIC_AS_MEMBAR is defined, all the .Xr atomic_ops 3 imply release and acquire operations, so the .Xr membar_acquire 3 and .Xr membar_release 3 are redundant. .Pp The combination of .Fn atomic_load_relaxed and .Xr membar_acquire 3 in that order is equivalent to .Fn atomic_load_acquire , and the combination of .Xr membar_release 3 and .Fn atomic_store_relaxed in that order is equivalent to .Fn atomic_store_release . .Sh EXAMPLES Maintaining lossy counters. These may lose some counts, because the read/modify/write cycle as a whole is not atomic. But this guarantees that the count will increase by at most one each time. In contrast, without atomic operations, in principle a write to a 32-bit counter might be torn into multiple smaller stores, which could appear to happen out of order from another CPU's perspective, leading to nonsensical counter readouts. (For frequent events, consider using per-CPU counters instead in practice.) .Bd -literal unsigned count; void record_event(void) { atomic_store_relaxed(&count, 1 + atomic_load_relaxed(&count)); } unsigned read_event_count(void) { return atomic_load_relaxed(&count); } .Ed .Pp Initialization barrier. .Bd -literal int ready; struct data d; void setup_and_notify(void) { setup_data(&d.things); atomic_store_release(&ready, 1); } void try_if_ready(void) { if (atomic_load_acquire(&ready)) do_stuff(d.things); } .Ed .Pp Publishing a pointer to the current snapshot of data. (Caller must arrange that only one call to .Li take_snapshot() happens at any given time; generally this should be done in coordination with .Xr pserialize 9 or similar to enable resource reclamation.) .Bd -literal struct data *current_d; void take_snapshot(void) { struct data *d = kmem_alloc(sizeof(*d)); d->things = ...; atomic_store_release(¤t_d, d); } struct data * get_snapshot(void) { return atomic_load_consume(¤t_d); } .Ed .Sh CODE REFERENCES .Pa sys/sys/atomic.h .Sh SEE ALSO .Xr atomic_ops 3 , .Xr membar_ops 3 , .Xr pserialize 9 .Sh HISTORY These atomic operations first appeared in .Nx 9.0 . .Sh CAVEATS C11 formally specifies that all subexpressions, except the left operands of the .Ql && , .Ql || , .Ql ?: , and .Ql \&, operators and the .Li kill_dependency() macro, carry dependencies for which .Dv memory_order_consume guarantees ordering, but most or all implementations to date simply treat .Dv memory_order_consume as .Dv memory_order_acquire and do not take advantage of data dependencies to elide costly memory barriers or load-acquire CPU instructions. .Pp Instead, we implement .Fn atomic_load_consume as .Fn atomic_load_relaxed followed by .Xr membar_datadep_consumer 3 , which is equivalent to .Xr membar_consumer 3 on DEC Alpha and .Xr __insn_barrier 3 elsewhere. .Sh BUGS Some idiot decided to call it .Em tearing , depriving us of the opportunity to say that atomic operations prevent fusion and .Em fission .