linux-kernel - Re: [PATCH 5/6] lib/refcount: Improve performance of generic REFCOUNT

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Date:   Fri, 2 Aug 2019 20:55:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Hanjun Guo <guohanjun@...wei.com>,
        Jan Glauber <jglauber@...vell.com>
Subject: Re: [PATCH 5/6] lib/refcount: Improve performance of generic
 REFCOUNT_FULL code

On Fri, Aug 02, 2019 at 11:09:59AM +0100, Will Deacon wrote:
>  static inline void refcount_add(int i, refcount_t *r)
>  {
> +	int old = atomic_fetch_add_relaxed(i, &r->refs);
> +
> +	WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n");
> +	if (unlikely(old <= 0 || old + i <= 0)) {
> +		refcount_set(r, REFCOUNT_SATURATED);
> +		WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n");
> +	}
>  }

That will trigger both WARNs when !old.