On Sat, 2013-04-06 at 10:10 +0200, Thomas Gleixner wrote: > Index: linux-stable/kernel/sched/clock.c > =================================================================== > --- linux.orig/kernel/sched/clock.c > +++ linux/kernel/sched/clock.c > @@ -176,10 +176,36 @@ static u64 sched_clock_remote(struct sch > u64 this_clock, remote_clock; > u64 *ptr, old_val, val; > > +#if BITS_PER_LONG != 64 > +again: > + /* > + * Careful here: The local and the remote clock values need to > + * be read out atomic as we need to compare the values and > + * then update either the local or the remote side. So the > + * cmpxchg64 below only protects one readout. > + * > + * We must reread via sched_clock_local() in the retry case on > + * 32bit as an NMI could use sched_clock_local() via the > + * tracer and hit between the readout of > + * the low32bit and the high 32bit portion. > + */ > + this_clock = sched_clock_local(my_scd); > + /* > + * We must enforce atomic readout on 32bit, otherwise the > + * update on the remote cpu can hit inbetween the readout of > + * the low32bit and the high 32bit portion. > + */ > + remote_clock = cmpxchg64(&scd->clock, 0, 0); > +#else > + /* > + * On 64bit the read of [my]scd->clock is atomic versus the > + * update, so we can avoid the above 32bit dance. > + */ > sched_clock_local(my_scd); > again: > this_clock = my_scd->clock; > remote_clock = scd->clock; > +#endif
Yeah, I like your version better.. just before reading your email I realized I could use the return value of sched_clock_local() to avoid a cmpxchg64() :-) That said, with this there's a subtle behavioural change between 32 and 64 bits with your patch; see how 32bit has sched_clock_local() inside the retry loop whereas 64 bit does not. Not sure we should worry about that though. Thanks! Acked-by: Peter Zijlstra <[email protected]> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
