Re: [PATCH 2/3] mm, notifier: Catch sleeping/blocking for !blockable

[Daniel Vetter]

On Thu, Nov 22, 2018 at 06:55:17PM +0000, Koenig, Christian wrote:
> Am 22.11.18 um 17:51 schrieb Daniel Vetter:
> > We need to make sure implementations don't cheat and don't have a
> > possible schedule/blocking point deeply burried where review can't
> > catch it.
> >
> > I'm not sure whether this is the best way to make sure all the
> > might_sleep() callsites trigger, and it's a bit ugly in the code flow.
> > But it gets the job done.
> >
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: "Christian König" <christian.koenig@xxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > ---
> >   mm/mmu_notifier.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > index 59e102589a25..4d282cfb296e 100644
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -185,7 +185,13 @@ int __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> >   	id = srcu_read_lock(&srcu);
> >   	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
> >   		if (mn->ops->invalidate_range_start) {
> > -			int _ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
> > +			int _ret;
> > +
> > +			if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !blockable)
> > +				preempt_disable();
> > +			_ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
> > +			if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !blockable)
> > +				preempt_enable();
> 
> Just for the sake of better documenting this how about adding this to 
> include/linux/kernel.h right next to might_sleep():
> 
> #define disallow_sleeping_if(cond)    for((cond) ? preempt_disable() : 
> (void)0; (cond); preempt_disable())
> 
> (Just from the back of my head, might contain peanuts and/or hints of 
> errors).

I think these magic for blocks aren't used in the kernel. goto breaks
them, and we use goto a lot. I think a disallow/allow_sleep() pair with
the conditional preept_disable/enable() calls would be nice though. I can
do that if the overall idea sticks.
-Daniel

> 
> Christian.
> 
> >   			if (_ret) {
> >   				pr_info("%pS callback failed with %d in %sblockable context.\n",
> >   						mn->ops->invalidate_range_start, _ret,
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch