Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 26, 2014 at 07:04:47PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 26, 2014 at 05:51:08PM +0100, Christian Borntraeger wrote:
> > > But this one was > giving users in field false positives.
> > 
> > So lets try to fix those, ok? If we cant, then tough luck.
> 
> Sure.
> I think the simplest way might be to make spinlock disable
> premption when CONFIG_DEBUG_ATOMIC_SLEEP is enabled.

Specifically maybe DEBUG_ATOMIC_SLEEP should select PREEMPT_COUNT?


> As a result, userspace access will fail and caller will
> get a nice error.
> 
> 
> 
> > But coming up with wrong statements is not helpful.
> 
> True. Sorry that I did that.
> 
> > > 
> > > The point is that *_user is safe with preempt off.
> > > It returns an error gracefully.
> > > It does not sleep.
> > > It does not trigger the scheduler in that context.
> > 
> > There are special cases where your statement is true. But its not in general.
> > copy_to_user might fault and that fault might sleep and reschedule.
> 
> Yes. But not if called inatomic.
> 
> 
> 
> > For example handle_mm_fault might go down to pud_alloc, pmd_alloc etc and all these functions could do an GFP_KERNEL allocation. Which might sleep. Which will schedule.
> > 
> > 
> > > 
> > > 
> > > David's patch makes it say it does, so it's wrong.
> > > 
> > > 
> > > 
> 
> Absolutely.
> I think you can already debug your case easily, by enabling CONFIG_PREEMPT.
> This seems counter-intuitive, and distro debug kernels don't seem to do this.
> 
> -- 
> MST