Re: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned

[Will Deacon]

On Mon, Feb 11, 2019 at 06:09:43PM +0100, Peter Zijlstra wrote:
> 
> Clarify the whole signed vs unsigned issue for atomic_t.
> 
> There has been enough confusion on this topic to warrant a few explicit
> words I feel.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  Documentation/atomic_t.txt | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> index 913396ac5824..dca3fb0554db 100644
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -56,6 +56,23 @@ The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
>    smp_mb__{before,after}_atomic()
>  
>  
> +TYPES (signed vs unsigned)
> +-----
> +
> +While atomic_t, atomic_long_t and atomic64_t use int, long and s64
> +respectively (for hysterical raisins), the kernel uses -fno-strict-overflow
> +(which implies -fwrapv) and defines signed overflow to behave like
> +2s-complement.
> +
> +Therefore, an explicitly unsigned variant of the atomic ops is strictly
> +unnecessary and we can simply cast, there is no UB.
> +
> +There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for
> +signed types.
> +
> +With this we also conform to the C/C++ _Atomic behaviour and things like
> +P1236R1.
> +

Acked-by: Will Deacon <will.deacon@xxxxxxx>

Will