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

From: Russell King <[email protected]>

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.

---

 Documentation/volatile-considered-harmful.txt |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff -puN Documentation/volatile-considered-harmful.txt~documentation-volatile-considered-harmfultxt-correct-cpu_relax-documentation 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:

_