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

Ming Lei
2018-05-22T08:51:44Z

On Mon, May 21, 2018 at 09:51:19PM -0600, Jens Axboe wrote:
> On May 21, 2018, at 9:47 PM, Ming Lei <ming.lei@redhat.com> 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.
>

It may not be something we are stuck at, and seems no alternatives for
this patchset.

It is a new requirement from NVMe, and Keith wants driver to complete
timed-out request in .timeout(). We never support that before for both
mq and non-mq code path.

Thanks,
Ming