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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 17, 2010 at 07:40:25PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Mon, May 17, 2010 at 06:00:25PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.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.

vhost uses only synchronize_rcu() and flush_work().  The existing
debugobjects tagging would therefore be unaware of the actual object,
instead tagging the rcu_head that synchronize_rcu() allocated on
the stack, and being out of the picture completely in the case of
flush_work().

Either way, RCU is completely unaware of exactly which structure is
being pushed through a grace period, so RCU's debugobjects tagging cannot
possibly conflict with any tagging that vhost does.

> > > 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.

Indeed!  Especially given the fact that RCU read-side critical sections
can be nested.  Which rcu_dereference() calls go with which 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?
> 
> 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.

We probably need vhost to add code at the end of the relevant RCU
read-side critical section checking that the pointers returned by
any rcu_dereference() calls still point to valid memory.  Don't get
me wrong, your approach could find bugs in which someone forgot to
remove the RCU-protected structure from a public list, but it could
not detect failure to wait a grace period between the time of removal
and the time of freeing.

							Thanx, Paul

> 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