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authorRussell King <rmk@arm.linux.org.uk>2010-03-23 13:35:16 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-03-24 16:31:20 -0700
commit091e635e6735fa4496c4a18e7e967b58e961303c (patch)
tree878136734a943d4a98f5f44f307aadfce523f00d /Documentation/volatile-considered-harmful.txt
parent7731d9a5d415414aa6903709453786d4a5ff57e4 (diff)
Documentation/volatile-considered-harmful.txt: correct cpu_relax() documentation
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>
Diffstat (limited to 'Documentation/volatile-considered-harmful.txt')
-rw-r--r--Documentation/volatile-considered-harmful.txt6
1 files changed, 3 insertions, 3 deletions
diff --git a/Documentation/volatile-considered-harmful.txt b/Documentation/volatile-considered-harmful.txt
index 991c26a6ef64..db0cb228d64a 100644
--- a/Documentation/volatile-considered-harmful.txt
+++ b/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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