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

Ming Lei
2018-05-22T03:47:28Z

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.

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

Thanks,
Ming