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Date: Mon, 24 Apr 2017 12:45:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, Eric Biggers <ebiggers3@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
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        linux-arch@vger.kernel.org, kernel-hardening@lists.openwall.com,
        PaX Team <pageexec@freemail.hu>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling
Message-ID: <20170424104539.qa2jva2jytwboeam@hirez.programming.kicks-ass.net>
References: <20170421220939.GA65363@beast>
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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;
> +}