Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited private command

[Peter Zijlstra <peterz@infradead.org>]

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.