Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

[David Laight]

On Fri, 10 Jan 2025 20:25:57 -0800
Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:

> Introduce functions to increase refcount but with a top limit above which
> they will fail to increase (the limit is inclusive). Setting the limit to
> INT_MAX indicates no limit.

This function has never worked as expected!
I've removed the update and added in the rest of the code.

> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index 35f039ecb272..5072ba99f05e 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -137,13 +137,23 @@ static inline unsigned int refcount_read(const refcount_t *r)
>  }
>  
>  static inline __must_check __signed_wrap
> -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
>  {
>  	int old = refcount_read(r);
>  
>  	do {
>  		if (!old)
>  			break;
>
>  	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
>  
>  	if (oldp)
>		*oldp = old;
?
>	if (unlikely(old < 0 || old + i < 0))
>		refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
>
>  	return old;
>  }

The saturate test just doesn't work as expected.
In C signed integer overflow is undefined (probably so that cpu that saturate/trap
signed overflow can be conformant) and gcc uses that to optimise code.

So if you compile (https://www.godbolt.org/z/WYWo84Weq):
int inc_wraps(int i)
{
    return i < 0 || i + 1 < 0;
}
the second test is optimised away.
I don't think the kernel compiles disable this optimisation.

	David