Re: [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks

[Will Deacon <will@kernel.org>]

On Tue, Feb 18, 2020 at 03:27:00PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 18, 2020 at 02:13:10PM +0100, Peter Zijlstra wrote:
> > (with the caveat that try_cmpxchg() doesn't seem available on !x86 -- I
> > should go fix that)
> 
> Completely untested (lemme go do that shortly), but something like so I
> suppose.
> 
> ---
> Subject: asm-generic/atomic: Add try_cmpxchg() fallbacks
> 
> Only x86 provides try_cmpxchg() outside of the atomic_t interfaces,
> provide generic fallbacks to create this interface from the widely
> available cmpxchg() function.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> diff --git a/include/linux/atomic-fallback.h b/include/linux/atomic-fallback.h
> index 656b5489b673..243f61d6c35f 100644
> --- a/include/linux/atomic-fallback.h
> +++ b/include/linux/atomic-fallback.h
> @@ -77,6 +77,50 @@
>  
>  #endif /* cmpxchg64_relaxed */
>  
> +#ifndef try_cmpxchg
> +#define try_cmpxchg(_ptr, _oldp, _new) \
> +({ \
> +	typeof(*ptr) ___r, ___o = *(_oldp); \

Probably worth pointing out that this will have the nasty behaviour
for volatile pointers that we're tackling for READ_ONCE. Obviously no
need to hold this up, but just mentioning it here in the hope that one
of us remembers to fix it later on.

> diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
> index b6c6f5d306a7..3c9be8d550e0 100755
> --- a/scripts/atomic/gen-atomic-fallback.sh
> +++ b/scripts/atomic/gen-atomic-fallback.sh
> @@ -140,6 +140,32 @@ cat <<EOF
>  EOF
>  }
>  
> +gen_try_cmpxchg_fallback()
> +{
> +	local order="$1"; shift;
> +
> +cat <<EOF
> +#ifndef try_cmpxchg${order}
> +#define try_cmpxchg${order}(_ptr, _oldp, _new) \\
> +({ \\
> +	typeof(*ptr) ___r, ___o = *(_oldp); \\
> +	___r = cmpxchg${order}((_ptr), ___o, (_new)); \\
> +	if (unlikely(___r != ___o)) \\
> +		*(_old) = ___r; \\

This doesn't compile because _old isn't declared. Probably best to avoid
evaluating _oldp twice though.

Will