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

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Message-ID: <20161120175207.GA4354@localhost>
Date:   Sun, 20 Nov 2016 23:52:07 +0600
From:   Alexnader Kuleshov <kuleshovmail@...il.com>
To:     open list <linux-kernel@...r.kernel.org>
Cc:     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: Question: goal of twice disabling of preemption in exception handlers

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.