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); } -- Jens Axboe -- 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