Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Thu, 25 Jul 2019 15:43:41 +0800 |
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On 2019/7/25 下午1:52, Michael S. Tsirkin wrote: > On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote: >> 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 > And I guess the deadlock is because GUP is taking mmu locks which are > taken on mmu notifier path, right?
Yes, but it's not the only lock. I don't remember the details, but I can confirm I meet issues with one or two other locks.
> How about we add a seqlock and take > that in invalidate callbacks? We can then drop the VQ lock before GUP, > and take it again immediately after. > > something like > if (!vq_meta_mapped(vq)) { > vq_meta_setup(&uaddrs); > mutex_unlock(vq->mutex) > vq_meta_map(&uaddrs);
The problem is the vq address could be changed at this time.
> mutex_lock(vq->mutex) > > /* recheck both sock->private_data and seqlock count. */ > if changed - bail out > } > > And also requires that VQ uaddrs is defined like this: > - writers must have both vq mutex and dev mutex > - readers must have either vq mutex or dev mutex > > > That's a big change though. For now, how about switching to a per-vq SRCU? > That is only a little bit more expensive than RCU, and we > can use synchronize_srcu_expedited. >
Consider we switch to use kfree_rcu(), what's the advantage of per-vq SRCU?
Thanks
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