linux-kernel - Re: [RFC] Disable lockref on arm64

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Message-ID: <201906150654.FF4400F7C8@keescook>
Date:   Sat, 15 Jun 2019 06:59:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Will Deacon <will.deacon@....com>,
        Jayachandran Chandrasekharan Nair <jnair@...vell.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        Jan Glauber <jglauber@...vell.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64

On Sat, Jun 15, 2019 at 10:47:19AM +0200, Ard Biesheuvel wrote:
> remaining question Will had was whether it makes sense to do the
> condition checks before doing the actual store, to avoid having a time
> window where the refcount assumes its illegal value. Since arm64 does
> not have memory operands, the instruction count wouldn't change, but
> it will definitely result in a performance hit on out-of-order CPUs.

What do the races end up looking like? Is it possible to have two
threads ordered in a way that a second thread could _un_saturate a
counter?

CPU 1                   CPU 2
inc()
  load INT_MAX-1
  about to overflow?
  yes
                        dec()
                          load INT_MAX-1
  set to INT_MAX
                          set to INT_MAX-2

Or would you use the same INT_MIN/2 saturation point done on x86?

As for performance, it should be easy to measure with the LKDTM test
to find out exactly the differences.

-- 
Kees Cook