linux-kernel - Re: Question: goal of twice disabling of preemption in exception handlers

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Message-ID: <20161127194525.GA4087@localhost.localdomain>
Date:   Mon, 28 Nov 2016 01:45:25 +0600
From:   Alexnader Kuleshov <kuleshovmail@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Alexnader Kuleshov <kuleshovmail@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Jan Beulich <jbeulich@...e.com>,
        Tony Luck <tony.luck@...el.com>,
        Jianyu Zhan <nasa4836@...il.com>
Subject: Re: Question: goal of twice disabling of preemption in exception
 handlers

On 11-27-16, Andy Lutomirski wrote:
> On Sun, Nov 20, 2016 at 9:52 AM, Alexnader Kuleshov
> <kuleshovmail@...il.com> wrote:
> > Hello everyone,
> >
> > Exception handlers which may run on IST stack disable and enable preemption
> > twice. For example do_int3() [1]. This one calls ist_enter() which
> > disables preemption with preempt_disable() every time without any conditions.
> > And later do_int3() calls preempt_disable() again [2] before do_trap().
> >
> > Of course we decrement preemption counter in the end of such exception
> > handlers twice too. But what's actual purpose of doing this two times?
> >
> > [1] https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/arch/x86/kernel/traps.c?id=refs/tags/next-20161117#n530
> > [2] https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/arch/x86/kernel/traps.c?id=refs/tags/next-20161117#n566
> >
> > Thank you.
> 
> You could submit a patch to get rid of the extra one if you like.

Thanks for reply. Will send a patch.