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

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Date:   Mon, 24 Apr 2017 10:32:50 +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:
> This patch ports the x86-specific atomic overflow handling from PaX's
> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version
> from PaX that eliminates the saturation race condition by resetting the
> atomic counter back to the INT_MAX saturation value on both overflow and
> underflow. To win a race, a system would have to have INT_MAX threads
> simultaneously overflow before the saturation handler runs.

And is this impossible? Highly unlikely I'll grant you, but absolutely
impossible?

Also, you forgot nr_cpus in your bound. Afaict the worst case here is
O(nr_tasks + 3*nr_cpus).

Because PaX does it, is not a correctness argument. And this really
wants one.