[Qemu-devel] Re: [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD

[Jan Kiszka]

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<address@hidden>
>> CC: Stefan Hajnoczi<address@hidden>
>> ---
>>
>> 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.

Jan

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