WARNING: suspicious RCU usage

[Russell King - ARM Linux]

On Sun, Dec 10, 2017 at 11:07:27AM -0800, Paul E. McKenney wrote:
> On Sun, Dec 10, 2017 at 12:00:12PM +0000, Russell King - ARM Linux wrote:
> > +Paul
> > 
> > Annoyingly, it looks like calling "complete()" from a dying CPU is
> > triggering the RCU usage warning.  From what I remember, this is an
> > old problem, and we still have no better solution for this other than
> > to persist with the warning.
> 
> I thought that this issue was resolved with tglx's use of IPIs from
> the outgoing CPU.  Or is this due to an additional complete() from the
> ARM code?  If so, could it also use tglx's IPI trick?

I don't think it was tglx's IPI trick, I've had code sitting in my tree
for a while for it, but it has its own set of problems which are not
resolvable:

1. it needs more IPIs than we have available on all platforms
2. there's some optional locking in the GIC driver that cause problems
   for the cpu dying path.

The concensus last time around was that the IPI solution is a non-
starter, so the seven year proven-reliable solution (disregarding the
RCU warning) persists because I don't think anyone came up with a
better solution.

> > I suspect the following lockdep warning is triggered by the RCU code
> > bringing the console semaphore into the mix of locks.
> 
> It does indeed look to me that it is quite possible that resolving
> the complete() issue would prevent the lockdep splat from appearing,
> which might in turn prevent acquisition of the console semaphore.

Yea, if only it was simple to resolve that.

-- 
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