linux-kernel - Re: [PATCH 2/2] arm64: preempt: Provide our own implementation of asm/preempt.h

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 28 Nov 2018 17:40:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Will Deacon <will.deacon@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>, rml@...h9.net,
        Thomas Gleixner <tglx@...utronix.de>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/2] arm64: preempt: Provide our own implementation of
 asm/preempt.h

On Wed, Nov 28, 2018 at 04:35:42PM +0100, Ard Biesheuvel wrote:
> On Tue, 27 Nov 2018 at 20:44, Will Deacon <will.deacon@....com> wrote:
> >
> > The asm-generic/preempt.h implementation doesn't make use of the
> > PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store
> > architectures which rely on the preempt_count word being unchanged across
> > an interrupt.
> >
> > However, since we're a 64-bit architecture and the preempt count is
> > only 32 bits wide, we can simply pack it next to the resched flag and
> > load the whole thing in one go, so that a dec-and-test operation doesn't
> > need to load twice.
> >
> 
> Since the actual preempt count is a lot narrower than 32 bits, x86
> just uses bit 31.
> 
> So what is the reason for using two different words?

See commit:

  ba1f14fbe709 ("sched: Remove PREEMPT_NEED_RESCHED from generic code")