| MEMBAR_OPS(3) | Library Functions Manual | MEMBAR_OPS(3) | 
membar_ops, membar_enter,
  membar_exit, membar_producer,
  membar_consumer,
  membar_datadep_consumer,
  membar_sync —
#include <sys/atomic.h>
void
  
  membar_enter(void);
void
  
  membar_exit(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_exit(), must pair with a
    barrier on another CPU, such as membar_enter(), 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_enter()membar_enter() will happen
      before all memory operations following it.
    An atomic read/modify/write operation
        (atomic_ops(3))
        followed by a membar_enter() implies a
        load-acquire operation in the language of C11.
WARNING: A load followed by
        membar_enter() does not imply
        a load-acquire operation, even though
        membar_exit() followed by a store implies a
        store-release operation; the symmetry of these names
        and asymmetry of the semantics is a historical mistake. In the
        NetBSD kernel, you can use
        atomic_load_acquire(9)
        for a load-acquire operation without any atomic
        read/modify/write.
membar_enter() is typically used in
        code that implements locking primitives to ensure that a lock protects
        its data, and is typically paired with
        membar_exit(); see below for an example.
membar_exit()membar_exit() will
      happen before any store that follows it.
    A membar_exit() followed by a store
        implies a store-release operation in the language of
        C11. For a regular store, rather than an atomic read/modify/write store,
        you should use
        atomic_store_release(9)
        instead of membar_exit() followed by the
      store.
membar_exit() is typically used in
        code that implements locking primitives to ensure that a lock protects
        its data, and is typically paired with
        membar_enter(). For example:
/* thread A */
obj->state.mumblefrotz = 42;
KASSERT(valid(&obj->state));
membar_exit();
obj->lock = 0;
/* thread B */
if (atomic_cas_uint(&obj->lock, 0, 1) != 0)
	return;
membar_enter();
KASSERT(valid(&obj->state));
obj->state.mumblefrotz--;
    
    In this example, if the
        atomic_cas_uint() operation in thread B
        witnesses the store obj->lock = 0 from thread
        A, then everything in thread A before the
        membar_exit() is guaranteed to happen before
        everything in thread B after the membar_enter(),
        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_exit() followed by a store,
        serving as a store-release operation, may also be
        paired with a subsequent load followed by
        membar_sync(), 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_exit().
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_exit() 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().
A load followed by membar_sync(),
        serving as a load-acquire operation, may also be
        paired with a prior membar_exit() followed by a
        store, serving as the corresponding store-release
        operation. However, you should use
        atomic_load_acquire(9)
        instead of
        load-then-membar_sync()
        if it is a regular load, or membar_enter()
        instead of membar_sync() if the load is in an
        atomic read/modify/write operation.
membar_ops functions first appeared in
  NetBSD 5.0. The data-dependent load barrier,
  membar_datadep_consumer(), first appeared in
  NetBSD 7.0.
| September 2, 2020 | NetBSD 9.4 |