Re: Re: Re: Re: [PATCH] rdma/siw: avoid smp_store_mb() on a u64

[Jason Gunthorpe]

On Fri, Jul 12, 2019 at 03:24:09PM +0000, Bernard Metzler wrote:
> 
> >To: "Bernard Metzler" <BMT@xxxxxxxxxxxxxx>
> >From: "Jason Gunthorpe" <jgg@xxxxxxxx>
> >Date: 07/12/2019 04:43PM
> >Cc: "Arnd Bergmann" <arnd@xxxxxxxx>, "Doug Ledford"
> ><dledford@xxxxxxxxxx>, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>,
> >linux-rdma@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> >Subject: [EXTERNAL] Re: Re: Re: [PATCH] rdma/siw: avoid
> >smp_store_mb() on a u64
> >
> >On Fri, Jul 12, 2019 at 02:35:50PM +0000, Bernard Metzler wrote:
> >
> >> >This looks wrong to me.. a userspace notification re-arm cannot be
> >> >lost, so have a split READ/TEST/WRITE sequence can't possibly
> >work?
> >> >
> >> >I'd expect an atomic test and clear here?
> >> 
> >> We cannot avoid the case that the application re-arms the
> >> CQ only after a CQE got placed. That is why folks are polling the
> >> CQ once after re-arming it - to make sure they do not miss the
> >> very last and single CQE which would have produced a CQ event.
> >
> >That is different, that is re-arm happing after a CQE placement and
> >this can't be fixed.
> >
> >What I said is that a re-arm from userspace cannot be lost. So you
> >can't blindly clear the arm flag with the WRITE_ONCE. It might be OK
> >beacuse of the if, but...
> >
> >It is just goofy to write it without a 'test and clear' atomic. If
> >the
> >writer side consumes the notify it should always be done atomically.
> >
> Hmmm, I don't yet get why we should test and clear atomically, if we
> clear anyway - is it because we want to avoid clearing a re-arm which
> happens just after testing and before clearing?

It is just clearer as to the intent.. 

Are you trying to optimize away an atomic or something? That might
work better as a dual counter scheme.

> Another complication -- test_and_set_bit() operates on a single
> bit, but we have to test two bits, and reset both, if one is
> set.

Why are two bits needed to represent armed and !armed?

Jason