Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

Andrea Parri
2019-06-29T18:09:23Z

On Sat, Jun 29, 2019 at 09:55:33AM -0700, Paul E. McKenney wrote:
> On Sat, Jun 29, 2019 at 05:12:36PM +0200, Andrea Parri wrote:
> > Hi Steve,
> >
> > > As Paul stated, interrupts are synchronization points. Archs can only
> > > play games with ordering when dealing with entities outside the CPU
> > > (devices and other CPUs). But if you have assembly that has two stores,
> > > and an interrupt comes in, the arch must guarantee that the stores are
> > > done in that order as the interrupt sees it.
> >
> > Hopefully I'm not derailing the conversation too much with my questions
> > ... but I was wondering if we had any documentation (or inline comments)
> > elaborating on this "interrupts are synchronization points"?
>
> I don't know of any, but I would suggest instead looking at something
> like the Hennessey and Patterson computer-architecture textbook.
>
> Please keep in mind that the rather detailed documentation on RCU is a
> bit of an outlier due to the fact that there are not so many textbooks
> that cover RCU. If we tried to replicate all of the relevant textbooks
> in the Documentation directory, it would be quite a large mess. ;-)

You know some developers considered it worth to develop formal specs in
order to better understand concepts such as "synchronization" and "IRQs
(processing)"! ... ;-) I still think that adding a few paragraphs (if
only in informal prose) to explain that "interrupts are synchronization
points" wouln't hurt. And you're right, I guess we may well start from
a reference to H&P...

Remark: we do have code which (while acknowledging that "interrupts are
synchronization points") doesn't quite seem to "believe it", c.f., e.g.,
kernel/sched/membarrier.c:ipi_mb(). So, I guess the follow-up question
would be "Would we better be (more) paranoid? ..."

Thanks,
Andrea