MEMBAR_OPS(9F) Kernel Functions for Drivers MEMBAR_OPS(9F)
NAME
membar_ops, membar_enter, membar_exit, membar_producer,
membar_consumer - memory access synchronization barrier operations
SYNOPSIS
#include <sys/atomic.h>
void membar_enter(
void);
void membar_exit(
void);
void membar_producer(
void);
void membar_consumer(
void);
DESCRIPTION
The
membar_enter() function is a generic memory barrier used during
lock entry. It is placed after the memory operation that acquires the
lock to guarantee that the lock protects its data. No stores from
after the memory barrier will reach visibility and no loads from
after the barrier will be resolved before the lock acquisition
reaches global visibility.
The
membar_exit() function is a generic memory barrier used during
lock exit. It is placed before the memory operation that releases the
lock to guarantee that the lock protects its data. All loads and
stores issued before the barrier will be resolved before the
subsequent lock update reaches visibility.
The
membar_enter() and
membar_exit() functions are used together to
allow regions of code to be in relaxed store order and then ensure
that the load or store order is maintained at a higher level. They
are useful in the implementation of mutex exclusion locks.
The
membar_producer() function arranges for all stores issued before
this point in the code to reach global visibility before any stores
that follow. This is useful in producer modules that update a data
item, then set a flag that it is available. The memory barrier
guarantees that the available flag is not visible earlier than the
updated data, thereby imposing store ordering.
The
membar_consumer() function arranges for all loads issued before
this point in the code to be completed before any subsequent loads.
This is useful in consumer modules that check if data is available
and read the data. The memory barrier guarantees that the data is not
sampled until after the available flag has been seen, thereby
imposing load ordering.
RETURN VALUES
No values are returned.
ERRORS
No errors are defined.
CONTEXT
These functions can be called from user, interrupt, or kernel
context.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Committed |
+--------------------+-----------------+
SEE ALSO
atomic_ops(3C),
attributes(7),
atomic_add(9F),
atomic_and(9F),
atomic_bits(9F),
atomic_cas(9F),
atomic_dec(9F),
atomic_inc(9F),
atomic_ops(9F),
atomic_or(9F),
atomic_swap(9F)NOTES
Atomic instructions (see
atomic_ops(9F)) ensure global visibility of
atomically-modified variables on completion. In a relaxed store order
system, this does not guarantee that the visibility of other
variables will be synchronized with the completion of the atomic
instruction. If such synchronization is required, memory barrier
instructions must be used.
January 16, 2006 MEMBAR_OPS(9F)
