Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Christian König <christian.koenig@xxxxxxx>]

Am 24.08.2018 um 14:03 schrieb Michal Hocko:
> On Fri 24-08-18 13:57:52, Christian König wrote:
> > Am 24.08.2018 um 13:52 schrieb Michal Hocko:
> > > On Fri 24-08-18 13:43:16, Christian König wrote:
> [...]
> > > > That won't work like this there might be multiple
> > > > invalidate_range_start()/invalidate_range_end() pairs open at the same time.
> > > > E.g. the lock might be taken recursively and that is illegal for a
> > > > rw_semaphore.
> > > I am not sure I follow. Are you saying that one invalidate_range might
> > > trigger another one from the same path?
> > No, but what can happen is:
> >
> > invalidate_range_start(A,B);
> > invalidate_range_start(C,D);
> > ...
> > invalidate_range_end(C,D);
> > invalidate_range_end(A,B);
> >
> > Grabbing the read lock twice would be illegal in this case.
> I am sorry but I still do not follow. What is the context the two are
> called from?
I don't have the slightest idea.

> Can you give me an example. I simply do not see it in the
> code, mostly because I am not familiar with it.
I'm neither.

We stumbled over that by pure observation and after discussing the 
problem with Jerome came up with this solution.
No idea where exactly that case comes from, but I can confirm that it 
indeed happens.
Regards,
Christian.

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