On 06.03.20 15:39, Paolo Bonzini wrote: > On 06/03/20 15:30, David Hildenbrand wrote: >>> Assuming we're only talking about CPU ioctls (seems like a good >>> approximation) maybe you could use start_exclusive/end_exclusive? The >>> current_cpu->in_exclusive_context assignments can be made conditional on >>> "if (current_cpu)". >>> >>> However that means you have to drop the BQL, see >>> process_queued_cpu_work. It may be a problem. >>> >> Yeah, start_exclusive() is expected to be called without the BQL, >> otherwise the other CPUs would not be able to make progress and can >> eventually be "caught". >> >> It's essentially the same reason why I can't use high-level >> pause_all_vcpus()/resume_all_vcpus(). Will drop the BQL which is very >> bad for resizing code. > > But any other synchronization primitive that you do which blocks all > vCPUs will have the same issue, otherwise you get a deadlock. This is essentially what this patch solves. The lock essentially blocks anybody from entering, but not leaving a KVM ioctl. An inhibitor only waits for all IOCTLs to be left. No other lock prohibits that, so I don't think there can ever be a deadlock. -- Thanks, David / dhildenb
