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

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Date:   Thu, 5 Jan 2023 13:34:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jani Nikula' <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>
CC:     Arnd Bergmann <arnd@...db.de>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        'Andrzej Hajda' <andrzej.hajda@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: RE: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of
 xchg

From: Jani Nikula
> Sent: 05 January 2023 13:28
> 
> On Thu, 05 Jan 2023, Daniel Vetter <daniel@...ll.ch> wrote:
> > On Mon, Dec 12, 2022 at 09:38:12AM +0000, David Laight wrote:
> >> From: Andrzej Hajda <andrzej.hajda@...el.com>
> >> > 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.

	David

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