Messages in this thread | | | Date | Fri, 29 Aug 2008 15:07:37 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] Remove stop_machine during module load |
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On Fri, Aug 29, 2008 at 11:23:30PM +0200, Andi Kleen wrote: > Hi Paul, > > Thanks for the excellent review. > > On Fri, Aug 29, 2008 at 01:44:57PM -0700, Paul E. McKenney wrote: > > > > OK, what about the read side? Not so good for __unlink_module() to yank > > That's independent from my patch isn't it? I don't think I'm changing > anything here. All of the issues you're pointing out are already > in the code (except for the missing read_barrier_depends() perhaps) > > I think the lockless users like oops or sysrq-t typically have preemption > disabled, so they should be ok regarding that.
Ah -- perhaps I was confusing preventing CPU hotplug with preventing stop_machine(). So disabling preemption holds off stop_machine()? Yep, looks that way.
> > the module out from under a reader. Therefore, all readers must either > > disable interrupts to block stop_machine() or must hold some sort of > > mutex that prevents modules from being unloaded. > > > > First, where the heck -is- the read side... > > > > o each_symbol() needs its list_for_each_entry() to become > > list_for_each_entry_rcu() and needs local_irq_disable() > > Ah that's needed for the Alpha barrier depends semantics, > right?
Yep! And to prevent compiler optimizations that could have the same effect.
> > Yet another approach would be to use call_rcu() to defer the > > various kfree() &c calls later in free_module. > > I think that would be a the better approach.
Or maybe just disable preemption around the remaining readers, preventing any stop_machine()-based deletions from being carried out during the searches.
(And here I call myself a fan of real-time response!!! But I suppose that stop_machine() is going to be pretty hard on realtime response in any case, so just don't mess with modules while your real-time application is running...)
Thanx, Paul
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