MEMBAR_OPS(3) Library Functions Manual MEMBAR_OPS(3)

membar_ops, membar_acquire, membar_release, membar_producer, membar_consumer, membar_datadep_consumer, membar_sync — memory ordering barriers

#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);

The 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 (), 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, 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.

()
Any load preceding membar_acquire() will happen before all memory operations following it.

A load followed by a () 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).

() 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()
All memory operations preceding membar_release() will happen before any store that follows it.

A () 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).

() 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, the load in () in thread B witnesses the store in () in thread A setting the reference count to zero, 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--;

() 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()
All stores preceding membar_producer() will happen before any stores following it.

() has no analogue in C11.

() 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()
All loads preceding membar_consumer() will complete before any loads after it.

() has no analogue in C11.

() 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()
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);

() 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.

() 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 () is a no-op on those CPUs.

membar_sync()
All memory operations preceding membar_sync() will happen before any memory operations following it.

() is a sequential consistency acquire/release barrier, analogous to atomic_thread_fence(memory_order_seq_cst) in C11.

() is typically paired with 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.

The following memory barriers are deprecated. They were imported from Solaris, which describes them as providing ordering relative to ‘lock acquisition’, but the documentation in NetBSD disagreed with the implementation and use on the semantics.

()
Originally documented as store-before-load/store, this was instead implemented as load-before-load/store on some platforms, which is what essentially all uses relied on. Now this is implemented as an alias for membar_sync() everywhere, meaning a full load/store-before-load/store sequential consistency barrier, in order to guarantee what the documentation claimed what the implementation actually did.

New code should use () 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.

()
Alias for membar_release(). This was originally meant to be paired with membar_enter().

New code should use () instead.

atomic_ops(3), atomic_loadstore(9), bus_dma(9), bus_space(9)

The 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 11.0