Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

[Oleg Nesterov]

On 10/18, Andy Lutomirski wrote:
>
> Oleg, the code in kernel/signal.c:
>
>                 preempt_disable();
>                 read_unlock(&tasklist_lock);
>                 preempt_enable_no_resched();
>                 freezable_schedule();
>
> looks bogus.  I don't get what it's trying to achieve with
> preempt_disable(), and I also don't see why no_resched does anything.
> Sure, it prevents a reschedule triggered during read_unlock() from
> causing a reschedule,

Yes. Lets suppose we remove preempt_disable/enable.

Debugger was already woken up, if it runs on the same CPU quite possibly
it will preemt the tracee. After that debugger will spin in wait_task_inactive(),
until it is in turn preempted or calls schedule_timeout(1), so that the tracee
(current) can finally call __schedule(preempt = F) and call deactivate_task() to
become inactive.

> but it doesn't prevent an interrupt immediately
> after the preempt_enable_no_resched() call from scheduling.

Yes, but this is less likely.

Oleg.