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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 15 Dec 2016 09:24:59 -0800
Message-ID: <CALCETrVSOVaTWC3GVwUpz-H_84c08Vxw+QEzDMa0odRCHT4BWA@mail.gmail.com>
Subject: Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra()
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Kyle Huey <khuey@kylehuey.com>,
        Andy Lutomirski <luto@kernel.org>
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On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> -       if (test_tsk_thread_flag(prev_p, TIF_BLOCKSTEP) ^
> -           test_tsk_thread_flag(next_p, TIF_BLOCKSTEP)) {
> +       tifn = task_thread_info(next_p)->flags;
> +       tifp = task_thread_info(prev_p)->flags;

Minor nit, but I think that a sufficiently clever compiler could
interpret this to mean "no one else is modifying these flags, so I can
do clever crazy things".  Wrapping these in READ_ONCE might be
helpful.