Date:	Mon, 17 May 2010 19:40:25 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	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

* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> On Mon, May 17, 2010 at 06:00:25PM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > > 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().
> > 
> > We can think of my recent debugobjects addition as a small state machine
> > that is described by the code that owns the objects. At each state
> > transition, the code passes the expected state as well as the next
> > state.
> > 
> > The current implementation can only keep track of a single "state" per
> > object at once. This should be extended to be able to count the number
> > RCU read side C.S. in flight that are accessing to an object.
> 
> Not a problem, as vhost doesn't use call_rcu().  So there won't be a
> conflict between different debugobjects views of the same memory.

Not quite sure I follow you here.

> 
> > We could use a hook in rcu_dereference (which knows about the object)
> > and a hook in rcu_read_unlock (which determines the end of valid object
> > use).
> > 
> > We should hook into rcu_assign_pointer() to detect RCU structure
> > privatization. It should put these objects in a "privatized" hash table.
> > 
> > We should also hook into synchronize_rcu/sched() to remove the
> > privatized structures from the privatized hash.
> > 
> > A hook in "kfree" (maybe a new rcu_free(void (fctptr*)(void *)) wrapper ?)
> > would call a debugobject hook that would lookup the "privatized" hash.
> > If it contains the object to free, we check if there are RCU read-side
> > C.S. in flight using this object at the same time, and show an error if
> > both are true.
> 
> I believe that we can't bury this into the RCU primitives, because
> rcu_read_unlock() doesn't know what objects were referenced in the
> RCU read-side critical section.

Well, if we can find a way to match a sequence of rcu_dereference
performed from a thread with the following rcu_read_unlock(), then we
might have the information we need. But we would have to somehow tie the
debugobject context to the thread context. That sounds too complex for
what we are trying to achieve here.

> 
> But perhaps we should be simply treating this as a use-after-free
> problem, so that RCU is not directly involved. Isn't that the standard
> use of debugobjects anyway?

OK so we could tie "rcu_dereference" do debugobjects, and free would be
a standard free. Yes, I think it could be done. It looks a bit like the
memory allocation debugging code. If we know that a certain
rcu_dereference always access dynamically allocated memory, we could
probably add some checks there based on the memory allocator debug
objects.

Thanks,

Mathieu


> 
> 							Thanx, Paul
> 
> > Thoughts ?
> > 
> > Mathieu
> > 
> > 
> > > 
> > > 						Thanx, Paul
> > 
> > -- 
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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