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MB(9) NetBSD Kernel Developer's Manual MB(9)
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by Kimmo Suominen.
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mb, mb_memory, mb_read, mb_write -- memory barriers
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 imple-
ment ``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 pro-
cessor 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 order-
ing 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.
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.
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().
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().
bus_dma(9), bus_space(9), mutex(9), rwlock(9)
The memory barrier primitives first appeared in NetBSD 5.0.
NetBSD 5.0 April 8, 2007 NetBSD 5.0