linux-kernel - Re: [PATCH v2 0/6] Rework REFCOUNT_FULL using atomic_fetch

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Message-ID: <20190828073052.GL2332@hirez.programming.kicks-ass.net>
Date:   Wed, 28 Aug 2019 09:30:52 +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 v2 0/6] Rework REFCOUNT_FULL using atomic_fetch_*
 operations

On Tue, Aug 27, 2019 at 05:31:58PM +0100, Will Deacon wrote:
> Will Deacon (6):
>   lib/refcount: Define constants for saturation and max refcount values
>   lib/refcount: Ensure integer operands are treated as signed
>   lib/refcount: Remove unused refcount_*_checked() variants
>   lib/refcount: Move bulk of REFCOUNT_FULL implementation into header
>   lib/refcount: Improve performance of generic REFCOUNT_FULL code
>   lib/refcount: Consolidate REFCOUNT_{MAX,SATURATED} definitions

So I'm not a fan; I itch at the whole racy nature of this thing and I
find the code less than obvious. Yet, I have to agree it is exceedingly
unlikely the race will ever actually happen, I just don't want to be the
one having to debug it.

I've not looked at the implementation much; does it do all the same
checks the FULL one does? The x86-asm one misses a few iirc, so if this
is similarly fast but has all the checks, it is in fact better.

Can't we make this a default !FULL implementation?