Christoph Lameter <cl@xxxxxxxxx> wrote: >On Thu, 25 Aug 2011, James Bottomley wrote: > >> On Thu, 2011-08-25 at 10:11 -0500, Christoph Lameter wrote: >> > On Thu, 25 Aug 2011, Peter Zijlstra wrote: >> > >> > > On Thu, 2011-08-18 at 14:40 -0700, Andrew Morton wrote: >> > > > >> > > > I think I'll apply it, as the call frequency is low (correct?) >and the >> > > > problem will correct itself as other architectures implement >their >> > > > atomic this_cpu_foo() operations. >> > > >> > > Which leads me to wonder, can anything but x86 implement that >this_cpu_* >> > > muck? I doubt any of the risk chips can actually do all this. >> > > Maybe Itanic, but then that seems to be dying fast. >> > >> > The cpu needs to have an RMW instruction that does something to a >> > variable relative to a register that points to the per cpu base. >> > >> > Thats generally possible. The problem is how expensive the RMW is >going to >> > be. >> >> Risc systems generally don't have a single instruction for this, >that's >> correct. Obviously we can do it as a non atomic sequence: read >> variable, compute relative, read, modify, write ... but there's >> absolutely no point hand crafting that in asm since the compiler can >> usually work it out nicely. And, of course, to have this atomic, we >> have to use locks, which ends up being very expensive. > >ARM seems to have these LDREX/STREX instructions for that purpose which >seem to be used for generating atomic instructions without lockes. I >guess >other RISC architectures have similar means of doing it? Arm isn't really risc. Most don't. However even with ldrex/strex you need two instructions for rmw. James -- Sent from my Android phone with K-9 Mail. Please excuse my brevity and top posting. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>