On Thu, 18 Mar 2010, akpm@xxxxxxxxxxxxxxxxxxxx wrote: > > 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. > > Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx> > Cc: <linux-arch@xxxxxxxxxxxxxxx> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
