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

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Date:	Wed, 19 Sep 2007 23:47:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
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 Wed, 19 Sep 2007 17:29:09 -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 16:41:04 -0400 "Dmitry Torokhov"
> > <dmitry.torokhov@...il.com> wrote:
> >
> > > > 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."
> >
> > That still does as it says in -rt. Its just that the interrupt handler
> > will be preemptible so the guarantees it gives are useless.
> 
> Please note "... including NMI and hardware-interrupt handlers ..."

-rt doesn't run interrupt handlers in hardware irq context anymore.

> >
> > > > 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?
> >
> > sure, but the cache eviction caused by running the driver will have
> > more impact than the added rcu_read_{,un}lock() calls.
> 
> Are you saying that adding rcu_read_{,un}lock() will help with cache
> eviction? How?

No, I'm saying that its noise compared to the cache eviction overhead
it causes for others.
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