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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
