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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 19 Sep 2007 15:49:24 -0400 "Dmitry Torokhov"
<dmitry.torokhov@gmail.com> wrote:

> On 9/19/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Wed, 19 Sep 2007 14:49:56 -0400 "Dmitry Torokhov"
> > <dmitry.torokhov@gmail.com> wrote:
> >
> > > On 9/19/07, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > > PS to previous -- any problem with inserting rcu_read_lock() and
> > > > rcu_read_unlock() around the portion of the IRQ handler that has
> > > > these accesses?
> > > >
> > >
> > > I guess I could but it is an extra lock that needs to be managed and
> > > given the fact that it is not really needed (other to make a newly
> > > developed tool happy) I am hestsant to do that.
> >
> > As is, these sites are a bug in -rt and we'll need to fix them anyway.
> >
> > As for the code you pointed me to, the i8042 driver, it seems to play
> > way to funny tricks for a simple 'slow' driver.
> 
> Even "slow" driver should try not to slow down the rest of the system
> if it can help it. I am sorry if the thing it does do not quite fit in
> with the changes you are proposing but it does not make the exeisting
> code invalid.
> 
> >
> > If you replace the spin_lock() + sync_sched(), with rcu_read_lock() +
> > rcu_call() it should work again without adding an extra lock.
> >
> 
> Except that I need spin_lock_irq for other reasons. I could take the
> same lock in write-side code and not use RCU at all but using RCU
> allows opening/closing input devices without slowing down interrupt
> handlers so why not use it?

If the IRQ handler does rcu_read_lock(),unlock() and the i8042_stop()
function does sync_rcu() instead of _sched(), it should be good again.
It will not affect anything else than the task that calls _stop(). And
even there the only change is that the sleep might be a tad longer.

I find it curious that a driver that is 'low performant' and does not
suffer lock contention pioneers locking schemes. I agree with
optimizing, but this is not the place to push the envelope.