On Wed, Sep 26, 2012 at 04:56:55PM +0200, Frederic Weisbecker wrote:
> 2012/9/25 Paul E. McKenney <paul...@linux.vnet.ibm.com>:
> > On Tue, Sep 25, 2012 at 01:59:26PM +0200, Frederic Weisbecker wrote:
> >> Given that we have:
> >>
> >> rcu_irq_enter()
> >> rcu_user_exit()
> >> rcu_user_enter()
> >> rcu_irq_exit()
> >
> > Indeed, the code to deal with irq misnestings won't like that at all.
> > And we are in the kernel between rcu_user_exit() and rcu_user_enter()
> > (right?), so we could in fact see irq misnestings.
>
> Exactly.
>
> >> And we already have rcu_user_exit_after_irq(), this starts to be confusing
> >> if we allow that nesting. Although if we find a solution that, in the end,
> >> merge rcu_user_exit() with rcu_user_exit_after_irq() and same for the
> >> enter version,
> >> this would probably be a good thing. Provided this doesn't involve some
> >> more
> >> complicated rdtp->dyntick_nesting trickies nor more overhead.
> >>
> >> Otherwise we could avoid to call rcu_user_* when we are in an irq. When
> >> we'll have
> >> the user_hooks layer, we can perhaps manage that from that place. For
> >> now may be we can return after in_interrupt() in the rcu user apis.
> >
> > This last sounds best.
>
> Ok.
>
> > My main concern is irq misnesting. We might need to do something ugly
> > like record the interrupt nesting level at rcu_user_exit() and restore
> > it at rcu_user_enter(). Sigh!!!
>
> Right, but that doesn't seem to apply in x86? I suggest we start
> simple and think
> about some wider solution when more architecture implement this.
Fair enough -- for one thing, we will better understand what is required
when the problems are actually encountered. Which will hopefully be
sooner rather than later. ;-)
Thanx, Paul
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