Re: [PATCH] cred - synchronize rcu before releasing cred

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

On Thu, Jul 29, 2010 at 09:34:20AM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > It is perfectly legal for an RCU callback to invoke call_rcu().  However,
> > this should be used -only- to wait for RCU readers.  If there are no
> > RCU readers, the callback might be re-invoked in very short order,
> > expecially on UP systems.
> > 
> > Or am I misunderstanding what you mean by "require call_rcu() to be
> > able to cope iwth requeueing"?
> 
> I mean for call_rcu() to be called on an object that's already been
> call_rcu()'d but not yet processed.

That would indeed be very bad!!!

> For example if struct cred gets its usage count reduced to 0, __put_cred()
> will call_rcu() it, but what happens if someone comes along and resurrects it
> by increasing its usage count again?  And what happens if the usage count is
> reduced back to zero and __put_cred() calls call_rcu() again before
> put_cred_rcu() has a chance to run?

Doing this would mess up RCU's internal data structures.  Mathieu
Desnoyers's recent debug changes (DEBUG_OBJECTS_RCU_HEAD) would catch
this sort of error.

							Thanx, Paul