linux-kernel - Re: Alloc refcount increments to fail

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Message-ID: <20190502154644.GV23075@ZenIV.linux.org.uk>
Date:   Thu, 2 May 2019 16:46:44 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: Alloc refcount increments to fail

On Thu, May 02, 2019 at 08:26:21AM -0700, Matthew Wilcox wrote:

> +/**
> + * refcount_try_inc - Increment a refcount if it's below INT_MAX
> + * @r: the refcount to increment
> + *
> + * Avoid the counter saturating by declining to increment the counter
> + * if it is more than halfway to saturation.
> + */
> +static inline __must_check bool refcount_try_inc(refcount_t *r)
> +{
> +	if (refcount_read(r) < 0)
> +		return false;
> +	refcount_inc(r);
> +	return true;
> +}

So two of those in parallel with have zero protection, won't they?