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

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Message-ID: <20070919204841.GH8666@linux.vnet.ibm.com>
Date:	Wed, 19 Sep 2007 13:48:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 Wed, Sep 19, 2007 at 03:49:24PM -0400, Dmitry Torokhov 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?

One approach would be to make rcu_read_held() check for in_interrupt()
or some such.  This would allow Dmitry's code to stay as it is, for the
moment at least.

							Thanx, Paul
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