On Fri, Sep 22, 2017 at 11:22:06AM +0800, Boqun Feng wrote: > The idea is in membarrier_private_expedited(), we go through all ->curr > on each CPU and > > 1) If it's a userspace task and its ->mm is matched, we send an ipi > > 2) If it's a kernel task, we skip > > (Because there will be a smp_mb() implied by mmdrop(), when it > switchs to userspace task). > > 3) If it's a userspace task and its ->mm is not matched, we take > the corresponding rq->lock and check rq->curr again, if its ->mm > matched, we send an ipi, otherwise we do nothing. > > (Because if we observe rq->curr is not matched with rq->lock > held, when a task having matched ->mm schedules in, the rq->lock > pairing along with the smp_mb__after_spinlock() will guarantee > it observes all memory ops before sys_membarrir()). 3) is an insta DoS.
