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

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Message-ID: <20190806120416.GB11627@ziepe.ca>
Date:   Tue, 6 Aug 2019 09:04:16 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier
 with worker

On Mon, Aug 05, 2019 at 12:20:45PM +0800, Jason Wang wrote:
> 
> On 2019/8/2 下午8:46, 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...

> > > 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.

> 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