linux-kernel - Re: [PATCH 0/6] Rework REFCOUNT_FULL using atomic_fetch

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Message-ID: <20190802184947.GC2349@hirez.programming.kicks-ass.net>
Date:   Fri, 2 Aug 2019 20:49:47 +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 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations

On Fri, Aug 02, 2019 at 11:09:54AM +0100, Will Deacon wrote:

> Although the revised implementation passes all of the lkdtm REFCOUNT
> tests, there is a race condition introduced by the deferred saturation
> whereby if INT_MIN + 2 tasks take a reference on a refcount at
> REFCOUNT_MAX and are each preempted between detecting overflow and
> writing the saturated value without being rescheduled, then another task
> may end up erroneously freeing the object when it drops the refcount and
> sees zero. It doesn't feel like a particularly realistic case to me, but
> I thought I should mention it in case somebody else knows better.

So my OCD has always found that hole objectionable. Also I suppose the
cmpxchg ones are simpler to understand.

Maybe make this fancy stuff depend on !FULL ?