On Thursday, 24 of April 2008, Mark Lord wrote: > Jens Axboe wrote: > > On Wed, Apr 23 2008, Mark Lord wrote: > >> Jens Axboe wrote: > >>> On Wed, Apr 23 2008, Mark Lord wrote: > >>> .. > >>>> The second bug, is that for the halt case at least, > >>>> nobody waits for the other CPU to actually halt > >>>> before continuing.. so we sometimes enter the shutdown > >>>> code while other CPUs are still active. > >>>> > >>>> This causes some machines to hang at shutdown, > >>>> unless CPU_HOTPLUG is configured and takes them offline > >>>> before we get here. > >>> I'm guessing there's a reason it doesn't pass '1' as the last argument, > >>> because that would fix that issue? > >> .. > >> > >> Undoubtedly -- perhaps the called CPU halts, and therefore cannot reply. :) > > > > Uhm yes, I guess stop_this_cpu() does exactly what the name implies :-) > > > >> But some kind of pre-halt ack, perhaps plus a short delay by the caller > >> after receipt of the ack, would probably suffice to kill that bug. > >> > >> But I really haven't studied this code enough to know, > >> other than that it historically has been a sticky area > >> to poke around in. > > > > Something like this will close the window to right up until the point > > where the other CPUs have 'almost' called halt(). > > > > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c > > index 5398385..94ec9bf 100644 > > --- a/arch/x86/kernel/smp.c > > +++ b/arch/x86/kernel/smp.c > > @@ -155,8 +155,9 @@ static void stop_this_cpu(void *dummy) > > /* > > * Remove this CPU: > > */ > > - cpu_clear(smp_processor_id(), cpu_online_map); > > disable_local_APIC(); > > + cpu_clear(smp_processor_id(), cpu_online_map); > > + smp_wmb(); > > if (hlt_works(smp_processor_id())) > > for (;;) halt(); > > for (;;); > > @@ -175,6 +176,12 @@ static void native_smp_send_stop(void) > > > > local_irq_save(flags); > > smp_call_function(stop_this_cpu, NULL, 0, 0); > > + > > + while (cpus_weight(cpu_online_map) > 1) { > > + cpu_relax(); > > + smp_rmb(); > > + } > > + > > disable_local_APIC(); > > local_irq_restore(flags); > > } > .. > > Yup, that looks like it oughta work consistently. > Now we just need to hear from some of the folks who > have danced around this code in the past. > > (added Pavel & Rafael to Cc:). Well, it looks sane to me, but I'm not really an expert here. Thanks, Rafael -- 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