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

[Kirill Tkhai]

Ð ÐÑÐ, 14/02/2014 Ð 12:21 +0000, Catalin Marinas ÐÐÑÐÑ:
> On Fri, Feb 14, 2014 at 11:16:09AM +0000, Kirill Tkhai wrote:
> > Ð ÐÑÐ, 14/02/2014 Ð 10:52 +0000, Catalin Marinas ÐÐÑÐÑ:
> > > 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?

Everything is right, the only case, which we have to worry, is
schedule_tail()

> 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.
> 

Kirill

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