BUG : PowerPC RCU: torture test failed with __stack_chk_fail

[Joel Fernandes]

This is amazing debugging Boqun, like a boss! One comment below:

> > > Or something simple I haven't thought of? :)
> >
> > At what points can r13 change?  Only when some particular functions are
> > called?
> >
>
> r13 is the local paca:
>
>         register struct paca_struct *local_paca asm("r13");
>
> , which is a pointer to percpu data.
>
> So if a task schedule from one CPU to anotehr CPU, the value gets
> changed.

It appears the whole issue, per your analysis, is that the stack
checking code in gcc should not cache or alias r13, and must read its
most up-to-date value during stack checking, as its value may have
changed during a migration to a new CPU.

Did I get that right?

IMO, even without a reproducer, gcc on PPC should just not do that,
that feels terribly broken for the kernel. I wonder what clang does,
I'll go poke around with compilerexplorer after lunch.

Adding +Peter Zijlstra as well to join the party as I have a feeling
he'll be interested. ;-)

thanks,

 - Joel