linux-kernel - Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to

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Date:   Thu, 15 Dec 2016 09:24:59 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kyle Huey <khuey@...ehuey.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch 1/3] x86/process: Optimize TIF checks in switch_to_extra()

On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@...utronix.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.