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| author | Russell King <rmk@arm.linux.org.uk> | 2010-03-23 13:35:16 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2010-03-24 16:31:20 -0700 |
| commit | 091e635e6735fa4496c4a18e7e967b58e961303c (patch) | |
| tree | 878136734a943d4a98f5f44f307aadfce523f00d /Documentation/volatile-considered-harmful.txt | |
| parent | 7731d9a5d415414aa6903709453786d4a5ff57e4 (diff) | |
| download | lwn-091e635e6735fa4496c4a18e7e967b58e961303c.tar.gz lwn-091e635e6735fa4496c4a18e7e967b58e961303c.zip | |
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.txt | 6 |
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: |