Eric Lee / smarc-fsl-linux-kernel

25 Mar, 2010

1 commit

  • 091e635e6   Documentation/volatile-considered-harmful.txt: correct cpu_relax() documentation ... Browse Code »

    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
    Cc:
    Acked-by: Linus Torvalds
    Signed-off-by: Andrew Morton
    Signed-off-by: Linus Torvalds

    Russell King
     

27 Jul, 2008

1 commit

24 Jun, 2007

1 commit