Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read_lock()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Sep 19, 2007 at 11:16:21AM -0400, Dmitry Torokhov wrote:
> On 9/19/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Wed, 19 Sep 2007 10:17:25 -0400 "Dmitry Torokhov"
> > <dmitry.torokhov@gmail.com> wrote:
> >
> > > Hi Peter,
> > >
> > > On 9/19/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > > Warn when rcu_dereference() is not used in combination with rcu_read_lock()
> > > >
> > >
> > > According to Paul it is fine to use RCU primitives (when accompanied
> > > with proper comments) when the read-size critical section is guarded
> > > by spin_lock_irqsave()/spin_lock_irqsrestore() instead of
> > > rcu_read_lock()/rcu_read_unlock() and writers synchronize with
> > > synchronize_sched(), not synchronize_rcu(). Your patch will trigger
> > > warnign on such valid usages.
> > >
> >
> > Sounds fragile to begin with. But you're right in that that is valid
> > for Linux as you know it. However in -rt most/all spinlocks are
> > converted to sleeping locks. In that case sync_sched() is not enough.
> 
> OK, then it goes beyond RCU... We need to come up with something that
> can be used to synchronize with IRQ handlers (quite often in driver
> code one needs to be sure that current invocation of IRQ handler
> completed before doing something). And once we have it splinlock + RCU
> users can just use that method.

But Peter's approach would not cause a problem here -- you wouldn't be
doing an rcu_dereference from within the IRQ handler in this case, right?

That said, we will need something to handle threaded interrupts, since
synchronize_sched() only waits for hardirq, NMI, SMI, etc., and not
threaded IRQs.

							Thanx, Paul