MEMBAR_OPS(3) | Library Functions Manual | MEMBAR_OPS(3) |
membar_ops
, membar_acquire
,
membar_release
,
membar_producer
,
membar_consumer
,
membar_datadep_consumer
,
membar_sync
—
#include <sys/atomic.h>
void
membar_acquire
(void);
void
membar_release
(void);
void
membar_producer
(void);
void
membar_consumer
(void);
void
membar_datadep_consumer
(void);
void
membar_sync
(void);
membar_ops
family of functions prevent reordering of
memory operations, as needed for synchronization in multiprocessor execution
environments that have relaxed load and store order.
In general, memory barriers must come in pairs — a barrier
on one CPU, such as membar_release
(), must pair with
a barrier on another CPU, such as membar_acquire
(),
in order to synchronize anything between the two CPUs. Code using
membar_ops
should generally be annotated with
comments identifying how they are paired.
membar_ops
affect only operations on
regular memory, not on device memory; see
bus_space(9) and
bus_dma(9) for
machine-independent interfaces to handling device memory and DMA operations
for device drivers.
Unlike C11, all memory operations — that
is, all loads and stores on regular memory — are affected by
membar_ops
, not just C11 atomic operations on
_Atomic-qualified objects.
membar_acquire
()membar_acquire
() will happen
before all memory operations following it.
A load followed by a membar_acquire
()
implies a load-acquire operation in the language of
C11. membar_acquire
() should only be used after
atomic read/modify/write, such as
atomic_cas_uint(3).
For regular loads, instead of x = *p;
membar_acquire()
, you should use x =
atomic_load_acquire(p)
.
membar_acquire
() is typically used in
code that implements locking primitives to ensure that a lock protects
its data, and is typically paired with
membar_release
(); see below for an example.
membar_release
()membar_release
()
will happen before any store that follows it.
A membar_release
() followed by a store
implies a store-release operation in the language of
C11. membar_release
() should only be used before
atomic read/modify/write, such as
atomic_inc_uint(3).
For regular stores, instead of membar_release(); *p =
x
, you should use atomic_store_release(p,
x)
.
membar_release
() is typically paired
with membar_acquire
(), and is typically used in
code that implements locking or reference counting primitives. Releasing
a lock or reference count should use
membar_release
(), and acquiring a lock or
handling an object after draining references should use
membar_acquire
(), so that whatever happened
before releasing will also have happened before acquiring. For
example:
/* thread A -- release a reference */ obj->state.mumblefrotz = 42; KASSERT(valid(&obj->state)); membar_release(); atomic_dec_uint(&obj->refcnt); /* * thread B -- busy-wait until last reference is released, * then lock it by setting refcnt to UINT_MAX */ while (atomic_cas_uint(&obj->refcnt, 0, -1) != 0) continue; membar_acquire(); KASSERT(valid(&obj->state)); obj->state.mumblefrotz--;
In this example, if the load in
atomic_cas_uint
() in thread B witnesses the
store in atomic_dec_uint
() in thread A setting
the reference count to zero, then everything in thread
A before the membar_release
() is guaranteed to
happen before everything in thread B after the
membar_acquire
(), as if the machine had
sequentially executed:
obj->state.mumblefrotz = 42; /* from thread A */ KASSERT(valid(&obj->state)); ... KASSERT(valid(&obj->state)); /* from thread B */ obj->state.mumblefrotz--;
membar_release
() followed by a store,
serving as a store-release operation, may also be
paired with a subsequent load followed by
membar_acquire
(), serving as the corresponding
load-acquire operation. However, you should use
atomic_store_release(9)
and
atomic_load_acquire(9)
instead in that situation, unless the store is an atomic
read/modify/write which requires a separate
membar_release
().
membar_producer
()membar_producer
() will happen
before any stores following it.
membar_producer
() has no analogue in
C11.
membar_producer
() is typically used in
code that produces data for read-only consumers which use
membar_consumer
(), such as
‘seqlocked’ snapshots of statistics; see below for an
example.
membar_consumer
()membar_consumer
() will
complete before any loads after it.
membar_consumer
() has no analogue in
C11.
membar_consumer
() is typically used in
code that reads data from producers which use
membar_producer
(), such as
‘seqlocked’ snapshots of statistics. For example:
struct { /* version number and in-progress bit */ unsigned seq; /* read-only statistics, too large for atomic load */ unsigned foo; int bar; uint64_t baz; } stats; /* producer (must be serialized, e.g. with mutex(9)) */ stats->seq |= 1; /* mark update in progress */ membar_producer(); stats->foo = count_foo(); stats->bar = measure_bar(); stats->baz = enumerate_baz(); membar_producer(); stats->seq++; /* bump version number */ /* consumer (in parallel w/ producer, other consumers) */ restart: while ((seq = stats->seq) & 1) /* wait for update */ SPINLOCK_BACKOFF_HOOK; membar_consumer(); foo = stats->foo; /* read out a candidate snapshot */ bar = stats->bar; baz = stats->baz; membar_consumer(); if (seq != stats->seq) /* try again if version changed */ goto restart;
membar_datadep_consumer
()membar_consumer
(), but limited to loads of
addresses dependent on prior loads, or ‘data-dependent’
loads:
int **pp, *p, v; p = *pp; membar_datadep_consumer(); v = *p; consume(v);
membar_datadep_consumer
() is typically
paired with membar_release
() by code that
initializes an object before publishing it. However, you should use
atomic_store_release(9)
and
atomic_load_consume(9)
instead, to avoid obscure edge cases in case the consumer is not
read-only.
membar_datadep_consumer
() does not
guarantee ordering of loads in branches, or
‘control-dependent’ loads — you must use
membar_consumer
() instead:
int *ok, *p, v; if (*ok) { membar_consumer(); v = *p; consume(v); }
Most CPUs do not reorder data-dependent loads (i.e., most CPUs
guarantee that cached values are not stale in that case), so
membar_datadep_consumer
() is a no-op on those
CPUs.
membar_sync
()membar_sync
() will
happen before any memory operations following it.
membar_sync
() is a sequential
consistency acquire/release barrier, analogous to
atomic_thread_fence(memory_order_seq_cst)
in
C11.
membar_sync
() is typically paired with
membar_sync
().
membar_sync
() is typically not needed
except in exotic synchronization schemes like Dekker's algorithm that
require store-before-load ordering. If you are tempted to reach for it,
see if there is another way to do what you're trying to do first.
membar_enter
()membar_sync
() everywhere, meaning a full
load/store-before-load/store sequential consistency barrier, in order to
guarantee what the documentation claimed and what the
implementation actually did.
New code should use membar_acquire
()
for load-before-load/store ordering, which is what most uses need, or
membar_sync
() for store-before-load/store
ordering, which typically only appears in exotic synchronization schemes
like Dekker's algorithm.
membar_exit
()membar_release
(). This was originally
meant to be paired with membar_enter
().
New code should use membar_release
()
instead.
membar_ops
functions first appeared in
NetBSD 5.0.
The data-dependent load barrier,
membar_datadep_consumer
(), first appeared in
NetBSD 7.0.
The membar_acquire
() and
membar_release
() functions first appeared, and the
membar_enter
() and
membar_exit
() functions were deprecated, in
NetBSD 10.0.
March 30, 2022 | NetBSD 10.1 |