I don't think that is the case for this particular issue. There may be other reported SMT bugs (still some instability) but here it has to be a pair of HTs:

I don't think that is the case for this particular issue. There may be other reported SMT bugs (still some instability) but here it has to be a pair of HTs: if one hyperthread is in a cpu-bound loop of any kind (can be in user mode), and the other hyperthread is returning from an interrupt via IRETQ ...