NAME

membar_ops, membar_enter, membar_exit, membar_producer, membar_consumer, membar_sync — memory access barrier operations

SYNOPSIS

#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);

DESCRIPTION

The membar_ops family of functions provide memory access barrier operations necessary for synchronization in multiprocessor execution environments that have relaxed load and store order.

membar_enter()

Any store preceding membar_enter() will reach global visibility before all loads and stores following it.

membar_enter() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_exit()

All loads and stores preceding membar_exit() will reach global visibility before any store that follows it.

membar_exit() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_producer()

All stores preceding the memory barrier will reach global visibility before any stores after the memory barrier reach global visibility.

membar_consumer()

All loads preceding the memory barrier will complete before any loads after the memory barrier complete.

membar_datadep_consumer()

Same as 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);

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()

All loads and stores preceding the memory barrier will complete and reach global visibility before any loads and stores after the memory barrier complete and reach global visibility.

SEE ALSO

atomic_ops(3)

HISTORY

The membar_ops functions first appeared in NetBSD 5.0. The data-dependent load barrier, membar_datadep_consumer(), first appeared in NetBSD 7.0.