linux-kernel - Re: [PATCH] x86/refcount: Implement fast refcount

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Date:   Mon, 24 Apr 2017 12:45:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jann@...jh.net>, davem@...emloft.net,
        linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
> +static __always_inline __must_check bool refcount_inc_not_zero(refcount_t *r)
> +{
> +	const int a = 1;
> +	const int u = 0;
> +	int c, old;
> +
> +	c = atomic_read(&(r->refs));
> +	for (;;) {
> +		if (unlikely(c == (u)))
> +			break;
> +		old = atomic_cmpxchg(&(r->refs), c, c + (a));

Please use atomic_try_cmpxchg(), that generates saner code.

> +		if (likely(old == c))
> +			break;
> +		c = old;
> +	}
> +	return c != u;
> +}