Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks forsimple self-deadlock

[Josh Triplett]

On Thu, Feb 02, 2012 at 12:42:06PM -0800, Paul E. McKenney wrote:
> On Thu, Feb 02, 2012 at 11:56:38AM -0800, Josh Triplett wrote:
> > On Thu, Feb 02, 2012 at 08:20:17AM -0800, Paul E. McKenney wrote:
> > > On Wed, Feb 01, 2012 at 04:55:54PM -0800, Josh Triplett wrote:
> > > > On Wed, Feb 01, 2012 at 11:41:21AM -0800, Paul E. McKenney wrote:
> > > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > > 
> > > > > It is illegal to have a grace period within a same-flavor RCU read-side
> > > > > critical section, so this commit adds lockdep-RCU checks to splat when
> > > > > such abuse is encountered.  This commit does not detect more elaborate
> > > > > RCU deadlock situations.  These situations might be a job for lockdep
> > > > > enhancements.
> > > > 
> > > > Since doing so also violates the prohibition on blocking within an RCU
> > > > read-side critical section, wouldn't it suffice to call might_sleep() or
> > > > equivalent, which also detects other problems?  (Obviously this doesn't
> > > > apply to SRCU, but it applies to the other variants of RCU.)
> > > 
> > > Yes, but...
> > > 
> > > The advantage of the lockdep-RCU splat is that it gives you a better
> > > hint as to where the RCU read-side critical section was entered, which
> > > is very helpful when tracking these down, especially when they are
> > > intermittent.
> > 
> > Ah, fair enough.
> > 
> > > And yes, I should also well check for the other variants of RCU read-side
> > > critical section (other than RCU).  Done.
> > 
> > Oh?  What hadn't you checked for?
> 
> Things like synchronize_sched() in rcu_read_lock() critical section
> and vice versa.

Ouch.  Good idea.

That also suggests another interesting possibility: lockdep could tag
pointers used in the flavor-specific rcu_dereference variants and
pointers used in the call_rcu variants to make sure nobody uses multiple
variants on the same pointer. :)  (Assuming we don't want
flavor-specific __rcu_* pointer tags.)

Speaking of which, could kfree_rcu require its argument to have the
__rcu type annotation?  We can't necessarily guarantee that for call_rcu
in all cases, but I think we can for kfree_rcu.

- Josh Triplett
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