Messages in this thread | | | From | David Laight <> | Subject | RE: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg | Date | Thu, 5 Jan 2023 14:41:43 +0000 |
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From: Daniel Vetter > Sent: 05 January 2023 14:13 ... > > > 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. > > Yeah this is my point for fetch_and_zero or any of the other proposals. > We're essentially replacing these two lines: > > var = some->pointer->chase; > some->pointer->chase = NULL; > > with a macro. C is verbose, and sometimes painfully so,
Try ADA or VHDL :-)
> if the pointer > chase is really to onerous then I think that should be refactored with a > meaningfully locally name variable, not fancy macros wrapped around to > golf a few characters away.
Provided 'var' is a local the compiler is pretty likely to only do the 'pointer chase' once. You can also do: var = NULL; swap(some->pointer->chase, var); and get pretty much the same object code.
> But what about swap() you ask? That one needs a temp variable, and it does > make sense to hide that in a ({}) block in a macro.
Sometimes, but not enough for the 'missed opportunity for swap()' message.
> But for the above two > lines I really don't see a point outside of obfuscated C contexts.
Indeed.
Isn't the suggested __xchg() in one of the 'reserved for implementation' namespaces - so shouldn't be a function that might be expected to be actually used.
David
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