On 2019/7/23 下午5:26, Michael S. Tsirkin wrote:

On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:

quoted

On 2019/7/23 下午4:10, Michael S. Tsirkin wrote:

quoted

On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:

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On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:

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Really let's just use kfree_rcu. It's way cleaner: fire and forget.

Looks not, you need rate limit the fire as you've figured out?

See the discussion that followed. Basically no, it's good enough
already and is only going to be better.

quoted

And in fact,
the synchronization is not even needed, does it help if I leave a comment to
explain?

Let's try to figure it out in the mail first. I'm pretty sure the
current logic is wrong.

Here is what the code what to achieve:

- The map was protected by RCU

- Writers are: MMU notifier invalidation callbacks, file operations (ioctls
etc), meta_prefetch (datapath)

- Readers are: memory accessor

Writer are synchronized through mmu_lock. RCU is used to synchronized
between writers and readers.

The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it
with readers (memory accessors) in the path of file operations. But in this
case, vq->mutex was already held, this means it has been serialized with
memory accessor. That's why I think it could be removed safely.

Anything I miss here?

So invalidate callbacks need to reset the map, and they do
not have vq mutex. How can they do this and free
the map safely? They need synchronize_rcu or kfree_rcu right?

Invalidation callbacks need but file operations (e.g ioctl) not.

quoted

And I worry somewhat that synchronize_rcu in an MMU notifier
is a problem, MMU notifiers are supposed to be quick:

Looks not, since it can allow to be blocked and lots of driver depends on
this. (E.g mmu_notifier_range_blockable()).

Right, they can block. So why don't we take a VQ mutex and be
done with it then? No RCU tricks.

This is how I want to go with RFC and V1. But I end up with deadlock 
between vq locks and some MM internal locks. So I decide to use RCU 
which is 100% under the control of vhost.

Thanks


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