On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: > > This must be a proper barrier, like a spinlock, mutex, or > > synchronize_rcu. > > > I start with synchronize_rcu() but both you and Michael raise some > concern. I've also idly wondered if calling synchronize_rcu() under the various mm locks is a deadlock situation. > Then I try spinlock and mutex: > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance > improvement. I think the topic here is correctness not performance improvement > 2) SRCU: full memory barrier requires on srcu_read_lock(), which still leads > little performance improvement > 3) mutex: a possible issue is need to wait for the page to be swapped in (is > this unacceptable ?), another issue is that we need hold vq lock during > range overlap check. I have a feeling that mmu notififers cannot safely become dependent on progress of swap without causing deadlock. You probably should avoid this. > > And, again, you can't re-invent a spinlock with open coding and get > > something better. > > So the question is if waiting for swap is considered to be unsuitable for > MMU notifiers. If not, it would simplify codes. If not, we still need to > figure out a possible solution. > > Btw, I come up another idea, that is to disable preemption when vhost thread > need to access the memory. Then register preempt notifier and if vhost > thread is preempted, we're sure no one will access the memory and can do the > cleanup. I think you should use the spinlock so at least the code is obviously functionally correct and worry about designing some properly justified performance change after. Jason
