On Tue, Mar 24, 2015 at 04:34:12PM +0100, Radim Krčmář wrote: > 2015-03-23 20:21-0300, Marcelo Tosatti: > > The following point: > > > > 2. per-CPU pvclock time info is updated if the > > underlying CPU changes. > > > > Is not true anymore since "KVM: x86: update pvclock area conditionally, > > on cpu migration". > > I think that the revert doesn't fix point 2.: "KVM: x86: update pvclock > [...]" changed the host to skip clock update on physical CPU change, but > guest's task migration notifier isn't tied to it at all. "per-CPU pvclock time info is updated if the underlying CPU changes" is the same as "always perform clock update on physical CPU change". That was a requirement for the original patch, to drop migration notifiers. > (Guest can have all tasks pinned, so the revert changed nothing.) > > > Add task migration notification back. > > > > Problem noticed by Andy Lutomirski. > > What is the problem? > > Thanks. The problem is this: T1) guest thread1 on vcpu1. T2) guest thread1 on vcpu2. T3) guest thread1 on vcpu1. Inside a pvclock read loop. Since the writes by hypervisor of pvclock area are not ordered, you cannot rely on version being updated _before_ the rest of pvclock data. (in the case above, "has the physical cpu changed" check, inside the guests thread1, obviously fails). -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html