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

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Date:	Wed, 19 Sep 2007 15:49:24 -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 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?

-- 
Dmitry
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