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

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Message-ID: <d120d5000709191149q3d850d07nd8bf864a6e797055@mail.gmail.com>
Date:	Wed, 19 Sep 2007 14:49:56 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	paulmck@...ux.vnet.ibm.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 9/19/07, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> On Wed, Sep 19, 2007 at 10:32:49AM -0700, Paul E. McKenney wrote:
> > On Wed, Sep 19, 2007 at 12:59:10PM -0400, Dmitry Torokhov wrote:
> > > On 9/19/07, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > > > On Wed, Sep 19, 2007 at 11:16:21AM -0400, Dmitry Torokhov wrote:
> > > > > On 9/19/07, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > > > > > On Wed, 19 Sep 2007 10:17:25 -0400 "Dmitry Torokhov"
> > > > > > <dmitry.torokhov@...il.com> wrote:
> > > > > >
> > > > > > > Hi Peter,
> > > > > > >
> > > > > > > On 9/19/07, Peter Zijlstra <a.p.zijlstra@...llo.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.
> > > >
> > > > But Peter's approach would not cause a problem here -- you wouldn't be
> > > > doing an rcu_dereference from within the IRQ handler in this case, right?
> > >
> > > Yes I do. Along with list_for_each_rcu().
> >
> > OK, in that case it does indeed need to be handled.
>
> 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.

-- 
Dmitry
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