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

Dmitry Torokhov
2007-09-19T16:59:20Z

On 9/19/07, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 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?
>

Yes I do. Along with list_for_each_rcu().

> 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
>

--
Dmitry