linux-kernel - Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read

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Message-ID: <d120d5000709191341p1c331e80qf7bad973bc899a0@mail.gmail.com>
Date:	Wed, 19 Sep 2007 16:41:04 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Nick Piggin" <nickpiggin@...oo.com.au>
Subject: Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read_lock()

On 9/19/07, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Wed, 19 Sep 2007 15:49:24 -0400 "Dmitry Torokhov"
> <dmitry.torokhov@...il.com> wrote:
>
> > On 9/19/07, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > > On Wed, 19 Sep 2007 14:49:56 -0400 "Dmitry Torokhov"
> > > <dmitry.torokhov@...il.com> wrote:
> > >
> > > > On 9/19/07, Paul E. McKenney <paulmck@...ux.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.

And the IRQ handler needs to do some extra job... Anyway, it looks -rt
breaks synchronize_sched() and needs to have it fixed:

"/**
 * synchronize_sched - block until all CPUs have exited any non-preemptive
 * kernel code sequences.
 *
 * This means that all preempt_disable code sequences, including NMI and
 * hardware-interrupt handlers, in progress on entry will have completed
 * before this primitive returns."

>
> 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.

Please realize that evey microsecond wasted on a 'low performant'
driver is taken from high performers and if we can help it why
shouldn't we?

-- 
Dmitry
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