Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Tue, 23 Jul 2019 21:31:35 +0800 |
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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: >> On 2019/7/23 下午4:10, Michael S. Tsirkin wrote: >>> On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote: >>>> 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? >>>> >>> 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. >> >> >>> 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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