Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states

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

On Tue, Jun 07, 2011 at 02:19:07AM +0200, Frederic Weisbecker wrote:
> On Mon, Jun 06, 2011 at 11:10:21AM -0700, Paul E. McKenney wrote:
> > commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date:   Tue May 24 08:31:09 2011 -0700
> > 
> >     rcu: Restore checks for blocking in RCU read-side critical sections
> >     
> >     Long ago, using TREE_RCU with PREEMPT would result in "scheduling
> >     while atomic" diagnostics if you blocked in an RCU read-side critical
> >     section.  However, PREEMPT now implies TREE_PREEMPT_RCU, which defeats
> >     this diagnostic.  This commit therefore adds a replacement diagnostic
> >     based on PROVE_RCU.
> >     
> >     Because rcu_lockdep_assert() and lockdep_rcu_dereference() are now being
> >     used for things that have nothing to do with rcu_dereference(), rename
> >     lockdep_rcu_dereference() to lockdep_rcu_suspicious() and add a third
> >     argument that is a string indicating what is suspicious.  This third
> >     argument is passed in from a new third argument to rcu_lockdep_assert().
> >     Update all calls to rcu_lockdep_assert() to add an informative third
> >     argument.
> >     
> >     Finally, add a pair of rcu_lockdep_assert() calls from within
> >     rcu_note_context_switch(), one complaining if a context switch occurs
> >     in an RCU-bh read-side critical section and another complaining if a
> >     context switch occurs in an RCU-sched read-side critical section.
> >     These are present only if the PROVE_RCU kernel parameter is enabled.
> >     
> >     Again, you must enable PROVE_RCU to see these new diagnostics.  But you
> >     are enabling PROVE_RCU to check out new RCU uses in any case, aren't you?
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> A little comment about this patch:
> 
> <snip>
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 88547c8..8b4b3da 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -153,6 +153,12 @@ void rcu_bh_qs(int cpu)
> >   */
> >  void rcu_note_context_switch(int cpu)
> >  {
> > +	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
> > +			   "Illegal context switch in RCU-bh"
> > +			   " read-side critical section");
> > +	rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
> > +			   "Illegal context switch in RCU-sched"
> > +			   " read-side critical section");
> 
> This looks like more a check to make inside might_sleep().
> It's better because might_sleep() triggers the check even if
> we don't actually go to sleep.

This does make quite a bit of sense.

> In fact I believe might_sleep() already does the job fine:
> 
> If !PREEMPT, might_sleep() detects that preemption is disabled
> by rcu_read_lock().

If !PREEMPT, isn't the preempt_disable() called by rcu_read_lock()
implemented as follows?

#define preempt_disable()               do { } while (0)

Unless I am missing something, __might_sleep() won't detect that.

> If PREEMPT, might_sleep() checks rcu_preempt_depth().

Agreed, for CONFIG_TREE_PREEMPT_RCU and CONFIG_TINY_PREEMPT_RCU,
the existing might_sleep() checks do cover it.

So I could export an rcu_might_sleep() or some such that contained
the above two rcu_lockdep_assert()s, but only if !PREEMPT_RCU.
If PREEMPT_RCU, rcu_might_sleep() would be a no-op.

Seem reasonable, or am I missing something?

							Thanx, Paul