Re: [PATCH 2/2] s390/mm,tlb: race of lazy TLB flush vs. recreationof TLB entries

[Catalin Marinas]

On Thu, Nov 14, 2013 at 08:10:07AM +0000, Martin Schwidefsky wrote:
> On Wed, 13 Nov 2013 16:16:35 +0000
> Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> 
> > On 13 November 2013 08:16, Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:
> > > diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h
> > > index 5d1f950..e91afeb 100644
> > > --- a/arch/s390/include/asm/mmu_context.h
> > > +++ b/arch/s390/include/asm/mmu_context.h
> > > @@ -48,13 +48,38 @@ static inline void update_mm(struct mm_struct *mm, struct task_struct *tsk)
> > >  static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> > >                              struct task_struct *tsk)
> > >  {
> > > -       cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
> > > -       update_mm(next, tsk);
> > > +       int cpu = smp_processor_id();
> > > +
> > > +       if (prev == next)
> > > +               return;
> > > +       if (atomic_inc_return(&next->context.attach_count) >> 16) {
> > > +               /* Delay update_mm until all TLB flushes are done. */
> > > +               set_tsk_thread_flag(tsk, TIF_TLB_WAIT);
> > > +       } else {
> > > +               cpumask_set_cpu(cpu, mm_cpumask(next));
> > > +               update_mm(next, tsk);
> > > +               if (next->context.flush_mm)
> > > +                       /* Flush pending TLBs */
> > > +                       __tlb_flush_mm(next);
> > > +       }
> > >         atomic_dec(&prev->context.attach_count);
> > >         WARN_ON(atomic_read(&prev->context.attach_count) < 0);
> > > -       atomic_inc(&next->context.attach_count);
> > > -       /* Check for TLBs not flushed yet */
> > > -       __tlb_flush_mm_lazy(next);
> > > +}
> > > +
> > > +#define finish_switch_mm finish_switch_mm
> > > +static inline void finish_switch_mm(struct mm_struct *mm,
> > > +                                   struct task_struct *tsk)
> > > +{
> > > +       if (!test_and_clear_tsk_thread_flag(tsk, TIF_TLB_WAIT))
> > > +               return;
> > > +
> > > +       while (atomic_read(&mm->context.attach_count) >> 16)
> > > +               cpu_relax();
> > > +
> > > +       cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
> > > +       update_mm(mm, tsk);
> > > +       if (mm->context.flush_mm)
> > > +               __tlb_flush_mm(mm);
> > >  }
> > 
> > Some care is needed here with preemption (we had this on arm and I
> > think we need a fix on arm64 as well). Basically you set TIF_TLB_WAIT
> > on a thread but you get preempted just before finish_switch_mm(). The
> > new thread has the same mm as the preempted on and switch_mm() exits
> > early without setting another flag. So finish_switch_mm() wouldn't do
> > anything but you still switched to the new mm. The fix is to make the
> > flag per mm rather than thread (see commit bdae73cd374e).
> 
> Interesting. For s390 I need to make sure that each task attaching an
> mm waits for the completion of concurrent TLB flush operations. If the
> scheduler does not switch the mm I don't care, the mm is still attached.

I assume the actual hardware mm switch happens via update_mm(). If you
have a context_switch() to a thread which requires an update_mm() but you
defer this until finish_switch_mm(), you may be preempted before the
hardware update. If the new context_switch() schedules a thread with the
same mm as the preempted one, you no longer call update_mm(). So the new
thread actually uses an old hardware mm.

-- 
Catalin
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