linux-kernel - Re: [patch 3/3] x86/process: Optimize TIF

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Message-ID: <CALCETrXZP-5xuOE0nWzAoa2FufMGS7bgKeQLdwS4eOhHhS225w@mail.gmail.com>
Date:   Fri, 16 Dec 2016 10:34:32 -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 3/3] x86/process: Optimize TIF_NOTSC switch

On Fri, Dec 16, 2016 at 12:50 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 15 Dec 2016, Andy Lutomirski wrote:
>> On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > +static inline void cr4_toggle_bits(unsigned long mask)
>> > +{
>> > +       unsigned long cr4;
>> > +
>> > +       cr4 = this_cpu_read(cpu_tlbstate.cr4);
>> > +       cr4 ^= mask;
>> > +       this_cpu_write(cpu_tlbstate.cr4, cr4);
>> > +       __write_cr4(cr4);
>> > +}
>>
>> This scares me for the same reason as BTF, although this should at
>> least be less fragile.  But how about:
>
> If that is fragile then all cr4 manipulation code is fragile because it
> relies on cpu_tlbstate.cr4. The TIF flag and that per cpu thing are kept in
> sync.

True.