On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:
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.
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?
Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(),
(just a little bit more hard to trigger):
AFAIK these never run in response to guest events.
So they can take very long and guests still won't crash.
What if guest manages to escape to qemu?
Thanks
Then it's going to be slow. Why do we care?
What we do not want is synchronize_rcu that guest is blocked on.
Ok, this looks like that I have some misunderstanding here of the reason
why synchronize_rcu() is not preferable in the path of ioctl. But in kvm
case, if rcu_expedited is set, it can triggers IPIs AFAIK.
Thanks
