Question: goal of twice disabling of preemption in exception handlers

[Alexnader Kuleshov]

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.