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Documentation/volatile-considered-harmful.txt: correct cpu_relax() do…

…cumentation

cpu_relax() is documented in volatile-considered-harmful.txt to be a
memory barrier.  However, everyone with the exception of Blackfin and
possibly ia64 defines cpu_relax() to be a compiler barrier.

Make the documentation reflect the general concensus.

Linus sayeth:

: I don't think it was ever the intention that it would be seen as anything
: but a compiler barrier, although it is obviously implied that it might
: well perform some per-architecture actions that have "memory barrier-like"
: semantics.
:
: After all, the whole and only point of the "cpu_relax()" thing is to tell
: the CPU that we're busy-looping on some event.
:
: And that "event" might be (and often is) about reading the same memory
: location over and over until it changes to what we want it to be.  So it's
: quite possible that on various architectures the "cpu_relax()" could be
: about making sure that such a tight loop on loads doesn't starve cache
: transactions, for example - and as such look a bit like a memory barrier
: from a CPU standpoint.
:
: But it's not meant to have any kind of architectural memory ordering
: semantics as far as the kernel is concerned - those must come from other
: sources.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: <linux-arch@vger.kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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1 parent 7731d9a commit 091e635e6735fa4496c4a18e7e967b58e961303c Russell King committed with Mar 23, 2010
Showing with 3 additions and 3 deletions.
  1. +3 −3 Documentation/volatile-considered-harmful.txt
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6 Documentation/volatile-considered-harmful.txt
@@ -63,9 +63,9 @@ way to perform a busy wait is:
cpu_relax();
The cpu_relax() call can lower CPU power consumption or yield to a
-hyperthreaded twin processor; it also happens to serve as a memory barrier,
-so, once again, volatile is unnecessary. Of course, busy-waiting is
-generally an anti-social act to begin with.
+hyperthreaded twin processor; it also happens to serve as a compiler
+barrier, so, once again, volatile is unnecessary. Of course, busy-
+waiting is generally an anti-social act to begin with.
There are still a few rare situations where volatile makes sense in the
kernel:

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