Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

[Jason Wang]

On 2019/8/6 äå8:04, Jason Gunthorpe wrote:
> On Mon, Aug 05, 2019 at 12:20:45PM +0800, Jason Wang wrote:
> > On 2019/8/2 äå8:46, Jason Gunthorpe wrote:
> > > 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.
> >
> > Maybe, that's why I suggest to use vhost_work_flush() which is much
> > lightweight can can achieve the same function. It can guarantee all previous
> > work has been processed after vhost_work_flush() return.
> If things are already running in a work, then yes, you can piggyback
> on the existing spinlocks inside the workqueue and be Ok
>
> However, if that work is doing any copy_from_user, then the flush
> becomes dependent on swap and it won't work again...


Yes it do copy_from_user(), so we can't do this.


> > > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance
> > > > improvement.
> > > I think the topic here is correctness not performance improvement>
>   
> > But the whole series is to speed up vhost.
> So? Starting with a whole bunch of crazy, possibly broken, locking and
> claiming a performance win is not reasonable.


Yes, I admit this patch is tricky, I'm not going to push this. Will post 
a V3.


> > Spinlock is correct but make the whole series meaningless consider it won't
> > bring any performance improvement.
> You can't invent a faster spinlock by opencoding some wild
> scheme. There is nothing special about the usage here, it needs a
> blocking lock, plain and simple.
>
> Jason


Will post V3. Let's see if you are happy with that version.

Thanks