NAME
mb,
mb_memory,
mb_read,
mb_write —
memory barriers
SYNOPSIS
#include <sys/lock.h>
void
mb_memory(
void);
void
mb_read(
void);
void
mb_write(
void);
DESCRIPTION
Many types of processor can execute instructions in a different order than
issued by the compiler or assembler. On a uniprocessor system, out of order
execution is transparent to the programmer, operating system and applications,
as the processor must ensure that it is self consistent.
On multiprocessor systems, out of order execution can present a problem where
locks are not used to guarantee atomicity of access, because loads and stores
issued by any given processor can appear on the system bus (and thus appear to
other processors) in an unpredictable order.
mb_memory(),
mb_read(), and
mb_write() can be used to control the order in which memory
accesses occur, and thus the order in which those accesses become visible to
other processors. They can be used to implement “lockless” access
to data structures where the necessary barrier conditions are well understood.
Memory barriers can be computationally expensive, as they are considered
“serializing” operations and may stall further execution until the
processor has drained internal buffers and re-synchronized.
The memory barrier primitives control only the order of memory access. They
provide no guarantee that stores have been flushed to the bus, or that loads
have been made from the bus.
The memory barrier primitives are guaranteed only to prevent reordering of
accesses to main memory. They do not provide any guarantee of ordering when
used with device memory (for example, loads or stores to or from a PCI
device). To guarantee ordering of access to device memory, the
bus_dma(9) and
bus_space(9) interfaces
should be used.
FUNCTIONS
-
-
- mb_memory()
- Issue a full memory barrier, ordering all memory accesses.
Causes all loads and stores preceding the call to
mb_memory() to complete before further memory accesses
can be made.
-
-
- mb_read()
- Issue a read memory barrier, ordering all loads from
memory. Causes all loads preceding the call to mb_read()
to complete before further loads can be made. Stores may be reordered
ahead of or behind a call to mb_read().
-
-
- mb_write()
- Issue a write memory barrier, ordering all stores to
memory. Causes all stores preceding the call to
mb_write() to complete before further stores can be
made. Loads may be reordered ahead of or behind a call to
mb_write().
SEE ALSO
__insn_barrier(3),
bus_dma(9),
bus_space(9),
mutex(9),
rwlock(9)
HISTORY
The memory barrier primitives first appeared in
NetBSD
5.0.