linux-kernel - Re: [PATCH] locking/atomic: atomic64: remove unusable atomics

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Message-ID: <YbD3f3CnTUkInNoP@FVFF77S0Q05N>
Date:   Wed, 8 Dec 2021 18:20:47 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Cc:     arnd@...db.de, boqun.feng@...il.com, lkp@...el.com, will@...nel.org
Subject: Re: [PATCH] locking/atomic: atomic64: remove unusable atomics

Peter, did you have any thoughts on this, or would you be happy to pick it as a
cleanup?

If there's anything on your queue that I can help to get out of the way, please
let me know!

Thanks,
Mark.

On Fri, Nov 26, 2021 at 11:59:23AM +0000, Mark Rutland wrote:
> The generic atomic64 implementation provides:
> 
> * atomic64_and_return()
> * atomic64_or_return()
> * atomic64_xor_return()
> 
> ... but none of these exist in the standard atomic64 API as described by
> scripts/atomic/atomics.tbl, and none of these have prototypes exposed by
> <asm-generic/atomic64.h>.
> 
> The lkp kernel test robot noted this results in warnings when building with
> W=1:
> 
>   lib/atomic64.c:82:5: warning: no previous prototype for 'generic_atomic64_and_return' [-Wmissing-prototypes]
> 
>   lib/atomic64.c:82:5: warning: no previous prototype for 'generic_atomic64_or_return' [-Wmissing-prototypes]
> 
>   lib/atomic64.c:82:5: warning: no previous prototype for 'generic_atomic64_xor_return' [-Wmissing-prototypes]
> 
> This appears to have been a thinko in commit:
> 
>   28aa2bda2211f432 ("locking/atomic: Implement atomic{,64,_long}_fetch_{add,sub,and,andnot,or,xor}{,_relaxed,_acquire,_release}()")
> 
> ... where we grouped add/sub separately from and/ox/xor, so that we could avoid
> implementing _return forms for the latter group, but forgot to remove
> ATOMIC64_OP_RETURN() for that group.
> 
> This doesn't cause any functional problem, but it's pointless to build code
> which cannot be used. Remove the unusable code. This does not affect add/sub,
> for which _return forms will still be built.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Link: https://lore.kernel.org/r/202111120712.RtQHZohY-lkp@intel.com
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will@...nel.org>
> ---
>  lib/atomic64.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/lib/atomic64.c b/lib/atomic64.c
> index 3df653994177..caf895789a1e 100644
> --- a/lib/atomic64.c
> +++ b/lib/atomic64.c
> @@ -118,7 +118,6 @@ ATOMIC64_OPS(sub, -=)
>  #undef ATOMIC64_OPS
>  #define ATOMIC64_OPS(op, c_op)						\
>  	ATOMIC64_OP(op, c_op)						\
> -	ATOMIC64_OP_RETURN(op, c_op)					\
>  	ATOMIC64_FETCH_OP(op, c_op)
>  
>  ATOMIC64_OPS(and, &=)
> @@ -127,7 +126,6 @@ ATOMIC64_OPS(xor, ^=)
>  
>  #undef ATOMIC64_OPS
>  #undef ATOMIC64_FETCH_OP
> -#undef ATOMIC64_OP_RETURN
>  #undef ATOMIC64_OP
>  
>  s64 generic_atomic64_dec_if_positive(atomic64_t *v)
> -- 
> 2.30.2
>