Re: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg

[Tvrtko Ursulin]

On 05/01/2023 13:34, David Laight wrote:
> From: Jani Nikula
> > Sent: 05 January 2023 13:28
> >
> > On Thu, 05 Jan 2023, Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > On Mon, Dec 12, 2022 at 09:38:12AM +0000, David Laight wrote:
> > > > From: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> > > > > Sent: 09 December 2022 15:49
> > > > >
> > > > > The pattern of setting variable with new value and returning old
> > > > > one is very common in kernel. Usually atomicity of the operation
> > > > > is not required, so xchg seems to be suboptimal and confusing in
> > > > > such cases. Since name xchg is already in use and __xchg is used
> > > > > in architecture code, proposition is to name the macro exchange.
> > > >
> > > > Dunno, if it is non-atomic then two separate assignment statements
> > > > is decidedly more obvious and needs less brain cells to process.
> > > > Otherwise someone will assume 'something clever' is going on
> > > > and the operation is atomic.
> > >
> > > Yes, this also my take. The i915 code that uses this to excess is decidely
> > > unreadable imo, and the macro should simply be replaced by open-coded
> > > versions.
> > >
> > > Not moved into shared headers where even more people can play funny games
> > > with it.
> >
> > My stand in i915 has been that the local fetch_and_zero() needs to
> > go. Either replaced by a common helper in core kernel headers, or open
> > coded, I personally don't care, but the local version can't stay.
> >
> > My rationale has been that fetch_and_zero() looks atomic and looks like
> > it comes from shared headers, but it's neither. It's deceptive. It
> > started small and harmless, but things like this just proliferate and
> > get copy-pasted all over the place.
> >
> > So here we are, with Andrzej looking to add the common helper. And the
> > same concerns crop up. What should it be called to make it clear that
> > it's not atomic? Is that possible?
>
> old_value = read_write(variable, new_value);
>
> But two statements are much clearer.

In a later thread there was more discussion on this and some new 
suggestions - exchange(), replace() or even take() sound fine to me. 
Last one is perhaps most specialized if it implies zeroing, which I at 
least assume it does.

All three are distant enough from atomic connotations of xchg. If that 
was a concern with __xchg, which I not sure it should be since there is 
"prior art" in the kernel for atomic vs non-atomic like set_bit and 
__set_bit.

My 2c, regardless of what name, that it is not something which is 
strictly needed, but a convenient syntactic sugar. (Exploded line counts 
with sometimes single use local variables are a bit meh.) And I am not 
really sure that open coding is more readable once the new pattern would 
be established. In short, if there can be swap there can be $insert_name 
too I guess.

Bonus points if needlessly atomic sites can be converted but identifying 
them is probably an exercise for a later phase.

Regards,

Tvrtko

P.S. FWIW my preference are either replace() or __xchg().