Messages in this thread | | | Date | Thu, 5 Jan 2023 14:13:45 +0000 | Subject | Re: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg | From | Tvrtko Ursulin <> |
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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@ffwll.ch> wrote: >>> On Mon, Dec 12, 2022 at 09:38:12AM +0000, David Laight wrote: >>>> From: Andrzej Hajda <andrzej.hajda@intel.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.
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().
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