Re: [PATCH] sched/core: Create new task with twice disabled preemption

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Feb 14, 2014 at 11:16:09AM +0000, Kirill Tkhai wrote:
> В Птн, 14/02/2014 в 10:52 +0000, Catalin Marinas пишет:
> > On Thu, Feb 13, 2014 at 09:32:22PM +0400, Kirill Tkhai wrote:
> > > Look at ARM64's finish_arch_post_lock_switch(). It looks a task
> > > must to not be preempted between switch_mm() and this function.
> > > But in case of new task this is possible.
> > 
> > We had a thread about this at the end of last year:
> > 
> > https://lkml.org/lkml/2013/11/15/82
> >
> > There is indeed a problem on arm64, something like this (and I think
> > s390 also needs a fix):
> > 
> > 1. switch_mm() via check_and_switch_context() defers the actual mm
> >    switch by setting TIF_SWITCH_MM
> > 2. the context switch is considered 'done' by the kernel before
> >    finish_arch_post_lock_switch() and therefore we can be preempted to a
> >    new thread before finish_arch_post_lock_switch()
> > 3. The new thread has the same mm as the preempted thread but we
> >    actually missed the mm switching in finish_arch_post_lock_switch()
> >    because TIF_SWITCH_MM is per thread rather than mm
> > 
> > > This is the problem I tried to solve. I don't know arm64, and I can't
> > > say how it is serious.
> > 
> > Have you managed to reproduce this? I don't say it doesn't exist, but I
> > want to make sure that any patch actually fixes it.
> 
> No, I have not tried. I found this place while analysing scheduler code.
> But it seems with the RT technics suggested previous message it's quite
> possible.

Now I think I confused myself. Looking through the __schedule() code,
context_switch() and therefore finish_arch_post_lock_switch() are called
with preemption disabled. So the scenario above cannot exist since the
current thread cannot be preempted between switch_mm() and
finish_arch_post_lock_switch(). Do I miss anything?

Now I get your point about schedule_tail() which calls
finish_task_switch() with a preempt count of 0. I'll get back to your
original patch.

Thanks.

-- 
Catalin