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Date:   Mon, 29 Jul 2019 10:44:38 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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Subject: Re: WARNING in __mmdrop
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On Mon, Jul 29, 2019 at 10:24:43PM +0800, Jason Wang wrote:
> 
> On 2019/7/29 下午4:59, Michael S. Tsirkin wrote:
> > On Mon, Jul 29, 2019 at 01:54:49PM +0800, Jason Wang wrote:
> > > On 2019/7/26 下午9:49, Michael S. Tsirkin wrote:
> > > > > > Ok, let me retry if necessary (but I do remember I end up with deadlocks
> > > > > > last try).
> > > > > Ok, I play a little with this. And it works so far. Will do more testing
> > > > > tomorrow.
> > > > > 
> > > > > One reason could be I switch to use get_user_pages_fast() to
> > > > > __get_user_pages_fast() which doesn't need mmap_sem.
> > > > > 
> > > > > Thanks
> > > > OK that sounds good. If we also set a flag to make
> > > > vhost_exceeds_weight exit, then I think it will be all good.
> > > 
> > > After some experiments, I came up two methods:
> > > 
> > > 1) switch to use vq->mutex, then we must take the vq lock during range
> > > checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting
> > > flags during weight check should work but it still can't address the worst
> > > case: wait for the page to be swapped in. Is this acceptable?
> > > 
> > > 2) using current RCU but replace synchronize_rcu() with vhost_work_flush().
> > > The worst case is the same as 1) but we can check range without holding any
> > > locks.
> > > 
> > > Which one did you prefer?
> > > 
> > > Thanks
> > I would rather we start with 1 and switch to 2 after we
> > can show some gain.
> > 
> > But the worst case needs to be addressed.
> 
> 
> Yes.
> 
> 
> > How about sending a signal to
> > the vhost thread?  We will need to fix up error handling (I think that
> > at the moment it will error out in that case, handling this as EFAULT -
> > and we don't want to drop packets if we can help it, and surely not
> > enter any error states.  In particular it might be especially tricky if
> > we wrote into userspace memory and are now trying to log the write.
> > I guess we can disable the optimization if log is enabled?).
> 
> 
> This may work but requires a lot of changes.

I agree.

> And actually it's the price of
> using vq mutex. 

Not sure what's meant here.

> Actually, the critical section should be rather small, e.g
> just inside memory accessors.

Also true.

> 
> I wonder whether or not just do synchronize our self like:
> 
> static void inline vhost_inc_vq_ref(struct vhost_virtqueue *vq)
> {
>         int ref = READ_ONCE(vq->ref);
> 
>         WRITE_ONCE(vq->ref, ref + 1);
> smp_rmb();
> }
> 
> static void inline vhost_dec_vq_ref(struct vhost_virtqueue *vq)
> {
>         int ref = READ_ONCE(vq->ref);
> 
> smp_wmb();
>         WRITE_ONCE(vq->ref, ref - 1);
> }
> 
> static void inline vhost_wait_for_ref(struct vhost_virtqueue *vq)
> {
>         while (READ_ONCE(vq->ref));
> mb();
> }

Looks good but I'd like to think of a strategy/existing lock that let us
block properly as opposed to spinning, that would be more friendly to
e.g. the realtime patch.

> 
> Or using smp_load_acquire()/smp_store_release() instead?
> 
> Thanks

These are cheaper on x86, yes.

> >