Re: [PATCH RFC tip/core/rcu 23/23] vhost: add __rcu annotations

[Michael S. Tsirkin]

On Mon, May 17, 2010 at 04:05:33PM -0700, Paul E. McKenney wrote:
> On Mon, May 17, 2010 at 06:00:25PM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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.
> 
> > 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.
> 
> 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?
> 
> 							Thanx, Paul

Well, it does not have to be freed, memory could be reused
for something else. I was just saying that it would be nice
to catch the class of errors which includes a missing sync.


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