linux-kernel - Re: [PATCH RFC tip/core/rcu 23/23] vhost: add _

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Date:	Mon, 17 May 2010 14:06:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, Arnd Bergmann <arnd@...ay.de.ibm.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH RFC tip/core/rcu 23/23] vhost: add __rcu annotations

On Mon, May 17, 2010 at 11:33:49PM +0300, Michael S. Tsirkin wrote:
> On Thu, May 13, 2010 at 08:23:40AM -0700, Paul E. McKenney wrote:
> > On Thu, May 13, 2010 at 03:07:23PM +0200, Peter Zijlstra wrote:
> > > On Wed, 2010-05-12 at 16:00 -0700, Paul E. McKenney wrote:
> > > > Any thoughts?  One approach would be to create a separate lockdep class
> > > > for vhost workqueue state, similar to the approach used in instrument
> > > > rcu_read_lock() and friends. 
> > > 
> > > workqueue_struct::lockdep_map, its held while executing worklets.
> > > 
> > > lock_is_held(&vhost_workqueue->lockdep_map), should do as you want.
> > 
> > Thank you, Peter!!!
> > 
> > 							Thanx, Paul
> 
> vhost in fact does flush_work rather than
> flush_workqueue, so while for now everything runs
> from vhost_workqueue in theory nothing would break
> if we use some other workqueue or even a combination
> thereof.
> 
> I guess when/if this happens, we could start by converting
> to _raw and then devise a solution.

If there are a small finite number of work queues involved, we can
easily do something like:

	#ifdef CONFIG_PROVE_RCU
	int in_vhost_workqueue(void)
	{
		return in_workqueue_context(vhost_workqueue) ||
		       in_workqueue_context(vhost_other_workqueue) ||
		       in_workqueue_context(yet_another_vhost_workqueue);
	}
	#endif /* #ifdef CONFIG_PROVE_RCU */

Seem reasonable?

> By the way what would be really nice is if we had a way
> to trap when rcu protected pointer is freed without a flush
> while some reader is running. Current annotation does not
> allow this, does it?

Right now, it does not, but I wonder if something like Thomas's and
Mathieu's debugobjects work could be brought to bear on this problem?
This would need to be implemented in vhost, as synchronize_rcu() has
no way to know what memory it is flushing, nor does flush_work().

						Thanx, Paul
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