On Mon, Jan 31, 2011 at 12:18 PM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: > On 2011-01-31 13:13, Stefan Hajnoczi wrote: >> On Mon, Jan 31, 2011 at 11:27 AM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: >>> On 2011-01-31 11:03, Avi Kivity wrote: >>>> On 01/27/2011 04:33 PM, Jan Kiszka wrote: >>>>> Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between >>>>> checking for exit_request on vcpu entry and timer signals arriving >>>>> before KVM starts to catch them. Plug it by blocking both timer related >>>>> signals also on !CONFIG_IOTHREAD and process those via signalfd. >>>>> >>>>> As this fix depends on real signalfd support (otherwise the timer >>>>> signals only kick the compat helper thread, and the main thread hangs), >>>>> we need to detect the invalid constellation and abort configure. >>>>> >>>>> Signed-off-by: Jan Kiszka<jan.kiszka@xxxxxxxxxxx> >>>>> CC: Stefan Hajnoczi<stefanha@xxxxxxxxxxxxxxxxxx> >>>>> --- >>>>> >>>>> I don't want to invest that much into !IOTHREAD anymore, so let's see if >>>>> the proposed catch&abort is acceptable. >>>>> >>>> >>>> I don't understand the dependency on signalfd. The normal way of doing >>>> things, either waiting for the signal in sigtimedwait() or in >>>> ioctl(KVM_RUN), works with SIGALRM just fine. >>> >>> And how would you be kicked out of the select() call if it is waiting >>> with a timeout? We only have a single thread here. >>> >>> The only alternative is Stefan's original proposal. But that required >>> fiddling with the signal mask twice per KVM_RUN. >> >> I think my original patch messed with the sigmask in the wrong place, >> as you mentioned doing it twice per KVM_RUN isn't a good idea. I >> wonder if we can enable SIGALRM only in blocking calls and guest code >> execution but without signalfd. It might be possible, I don't see an >> immediate problem with doing that, we might have to use pselect(2) or >> similar in a few places. > > My main concern about alternative approaches is that IOTHREAD is about > to become the default, and hardly anyone (of the few upstream KVM users) > will run without it in the foreseeable future. The next step will be the > removal of any !CONFIG_IOTHREAD section. So, how much do we want to > invest here (provided my proposal has not remaining issues)? Yes, you're right. I'm not volunteering to dig more into this, the best case would be to switch to a non-I/O thread world that works for everybody. Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
