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Date: Wed, 19 Sep 2007 17:25:37 +0200
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: "Dmitry Torokhov" <dmitry.torokhov@gmail.com>
Cc: linux-kernel@vger.kernel.org,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       "Ingo Molnar" <mingo@elte.hu>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "Nick Piggin" <nickpiggin@yahoo.com.au>
Subject: Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs
 rcu_read_lock()
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On Wed, 19 Sep 2007 11:16:21 -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 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.

Sound like you want a completion or workqueue.