• Russell King's avatar
    Documentation/volatile-considered-harmful.txt: correct cpu_relax() documentation · 091e635e
    Russell King authored
    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: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    Cc: <linux-arch@vger.kernel.org>
    Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
volatile-considered-harmful.txt 5.56 KB