2016-11-20 17:52:19

by Alexander Kuleshov

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.

2016-11-27 18:58:00

by Andy Lutomirski

Subject: Re: Question: goal of twice disabling of preemption in exception handlers

On Sun, Nov 20, 2016 at 9:52 AM, Alexnader Kuleshov
<[email protected]> 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.

2016-11-27 19:45:43

by Alexander Kuleshov

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
> <[email protected]> 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.