Re: [RFC PATCH 3/3] blk-mq: Remove generation seqeunce

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On May 21, 2018, at 9:47 PM, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> 
>> On Mon, May 21, 2018 at 09:16:33PM -0600, Jens Axboe wrote:
>>> On 5/21/18 8:49 PM, Ming Lei wrote:
>>>> On Mon, May 21, 2018 at 05:11:31PM -0600, Keith Busch wrote:
>>>> This patch simplifies the timeout handling by relying on the request
>>>> reference counting to ensure the iterator is operating on an inflight
>>> 
>>> The reference counting isn't free, what is the real benefit in this way?
>> 
>> Neither is the current scheme and locking, and this is a hell of a lot
>> simpler. I'd get rid of the kref stuff and just do a simple
>> atomic_dec_and_test(). Most of the time we should be uncontended on
>> that, which would make it pretty darn cheap. I'd be surprised if it
>> wasn't better than the current alternatives.
> 
> The explicit memory barriers by atomic_dec_and_test() isn't free.

I’m not saying it’s free. Neither is our current synchronization.

> Also the double completion issue need to be fixed before discussing
> this approach further.

Certainly. Also not saying that the current patch is perfect. But it’s a lot more palatable than the alternatives, hence my interest. And I’d like for this issue to get solved, we seem to be a bit stuck atm. 


I’ll take a closer look tomorrow. 






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