Commit 6bc39274 authored by David Howells's avatar David Howells Committed by Linus Torvalds
Browse files

[PATCH] Corrections to memory barrier doc



Apply some small corrections to the memory barrier document, as contributed by:

	Christoph Lameter <clameter@sgi.com>
	Kirill Smelkov <kirr@mns.spb.ru>
	Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 6f84be84
......@@ -287,7 +287,7 @@ Memory barriers come in four basic varieties:
A write barrier is a partial ordering on stores only; it is not required
to have any effect on loads.
A CPU can be viewed as as commiting a sequence of store operations to the
A CPU can be viewed as committing a sequence of store operations to the
memory system as time progresses. All stores before a write barrier will
occur in the sequence _before_ all the stores after the write barrier.
......@@ -418,7 +418,7 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
indirect effect will be the order in which the second CPU sees the effects
of the first CPU's accesses occur, but see the next point:
(*) There is no guarantee that the a CPU will see the correct order of effects
(*) There is no guarantee that a CPU will see the correct order of effects
from a second CPU's accesses, even _if_ the second CPU uses a memory
barrier, unless the first CPU _also_ uses a matching memory barrier (see
the subsection on "SMP Barrier Pairing").
......@@ -489,7 +489,7 @@ lines. The pointer P might be stored in an odd-numbered cache line, and the
variable B might be stored in an even-numbered cache line. Then, if the
even-numbered bank of the reading CPU's cache is extremely busy while the
odd-numbered bank is idle, one can see the new value of the pointer P (&B),
but the old value of the variable B (1).
but the old value of the variable B (2).
Another example of where data dependency barriers might by required is where a
......@@ -749,7 +749,7 @@ some effectively random order, despite the write barrier issued by CPU 1:
: :
If, however, a read barrier were to be placed between the load of E and the
If, however, a read barrier were to be placed between the load of B and the
load of A on CPU 2:
CPU 1 CPU 2
......@@ -1466,9 +1466,8 @@ instruction itself is complete.
On a UP system - where this wouldn't be a problem - the smp_mb() is just a
compiler barrier, thus making sure the compiler emits the instructions in the
right order without actually intervening in the CPU. Since there there's only
one CPU, that CPU's dependency ordering logic will take care of everything
else.
right order without actually intervening in the CPU. Since there's only one
CPU, that CPU's dependency ordering logic will take care of everything else.
ATOMIC OPERATIONS
......@@ -1645,9 +1644,9 @@ functions:
The PCI bus, amongst others, defines an I/O space concept - which on such
CPUs as i386 and x86_64 cpus readily maps to the CPU's concept of I/O
space. However, it may also mapped as a virtual I/O space in the CPU's
memory map, particularly on those CPUs that don't support alternate
I/O spaces.
space. However, it may also be mapped as a virtual I/O space in the CPU's
memory map, particularly on those CPUs that don't support alternate I/O
spaces.
Accesses to this space may be fully synchronous (as on i386), but
intermediary bridges (such as the PCI host bridge) may not fully honour
......
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