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

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Date:	Thu, 20 Sep 2007 13:31:35 -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 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.

OK, then what is the purpose of synchronize_sched() in -rt?

You really need to provide users with a replacement. There are several
drivers that use it and for example r8169 is not what you'd call a
'low performer'.

I guess I can switch i8042 to use synchronize_irq(). That still works
in -rt, doesn't it? That still leaves atkbd...

>
> > >
> > > > > 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.
>

What about udelay(10)? It is probably also a noise but we shoudl not
go and sprinkle it through drivers, should we? ;)

-- 
Dmitry
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