[V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

[河合英宏 / KAWAI，HIDEHIRO]

> From: Peter Zijlstra [mailto:peterz at infradead.org]
> On Mon, Aug 31, 2015 at 08:53:11AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > > I understand your question.  I don't intend to permit the recursive
> > > call of crash_kexec() as for 'old_cpu != this_cpu' check.  That is
> > > needed for the case of panic() --> crash_kexec().  Since panic_cpu has
> > > already been set to this_cpu in panic() (please see PATCH 1/4), no one
> > > can run crash_kexec() without 'old_cpu != this_cpu' check.
> > >
> > > If you don't like this check, I would also be able to handle this case
> > > like below:
> > >
> > > crash_kexec()
> > > {
> > > 	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > > 	if (old_cpu != -1)
> > > 		return;
> > >
> > > 	__crash_kexec();
> > > }
> > >
> > > panic()
> > > {
> > > 	atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > > 	__crash_kexec();
> > > ...
> > >
> >
> > Is that OK?
> 
> I suppose so, but I think me getting confused means comments can be
> added/improved.

OK, I'll improve comments and description in the next version.

Thanks!

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group