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SubjectRe: WARNING in __mmdrop

On 2019/7/26 下午8:53, Jason Wang wrote:
> On 2019/7/26 下午8:38, Michael S. Tsirkin wrote:
>> On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote:
>>> On 2019/7/26 下午7:49, Michael S. Tsirkin wrote:
>>>> On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote:
>>>>> On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:
>>>>>>> Exactly, and that's the reason actually I use synchronize_rcu()
>>>>>>> there.
>>>>>>> So the concern is still the possible synchronize_expedited()?
>>>>>> I think synchronize_srcu_expedited.
>>>>>> synchronize_expedited sends lots of IPI and is bad for realtime VMs.
>>>>>>> Can I do this
>>>>>>> on through another series on top of the incoming V2?
>>>>>>> Thanks
>>>>>> The question is this: is this still a gain if we switch to the
>>>>>> more expensive srcu? If yes then we can keep the feature on,
>>>>> I think we only care about the cost on srcu_read_lock() which
>>>>> looks pretty
>>>>> tiny form my point of view. Which is basically a READ_ONCE() +
>>>>> WRITE_ONCE().
>>>>> Of course I can benchmark to see the difference.
>>>>>> if not we'll put it off until next release and think
>>>>>> of better solutions. rcu->srcu is just a find and replace,
>>>>>> don't see why we need to defer that. can be a separate patch
>>>>>> for sure, but we need to know how well it works.
>>>>> I think I get here, let me try to do that in V2 and let's see the
>>>>> numbers.
>>>>> Thanks
>>> It looks to me for tree rcu, its srcu_read_lock() have a mb() which
>>> is too
>>> expensive for us.
>> I will try to ponder using vq lock in some way.
>> Maybe with trylock somehow ...
> 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

One reason could be I switch to use get_user_pages_fast() to
__get_user_pages_fast() which doesn't need mmap_sem.


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