Re: [RFC GIT PULL] softirq: Consolidation and stack overrun fix

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Sat, 2013-09-21 at 19:01 -0700, H. Peter Anvin wrote:
> On 09/21/2013 02:45 PM, Benjamin Herrenschmidt wrote:
> > 
> > I really don't like the "larger" irq stack ... probably because I can't
> > make it work easily :-) See my previous comment about how we get to
> > thread_info on ppc.
> > 
> 
> For the record, I intend to remove thread_info from the stack on x86 and
> instead merge it with task_struct as a single structure pointed to with
> a percpu variable.

Last I looked, our per-cpu codegen was pretty poor... but then we have
this "PACA" (somewhat arch specific per-cpu blob that is separate from
the rest of per-cpu because of a mix of historical reasons and the fact
that it has to be allocated in a specific part of memory at boot time)
which we point to directly via a GPR, so we could point to it via PACA.

How do you do your per-cpu on x86 ? On powerpc we struggle because we
try to dedicate a register (r13) to this PACA (the per-cpu offset hangs
off it), but we constantly run into issues where gcc copies r13 to
another register and then indexes off that, even accross
preempt_enable/disable sections, or worst such as saving/restoring from
the stack. We can't seem to get the compiler to treat it appropriately
as volatile.

Ben.