Re: [PATCH tip/core/rcu 04/28] rcu: Restore checks for blocking in RCU read-side critical sections

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

On Thu, Jun 09, 2011 at 01:28:35AM +0200, Frederic Weisbecker wrote:
> On Wed, Jun 08, 2011 at 12:29:43PM -0700, Paul E. McKenney wrote:
> > 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>
> 
> So, do you think we can get rid of this patch now that we are going to have CONFIG_ATOMIC_SLEEP
> working everywhere?
> 
> The last remaining piece we need is to check rcu_preempt_depth() from schedule_debug(),
> which does a kind of lightweight might_sleep() check alike.

I believe that we need them both.  Your patch provides a lightweight
check.  Mine is way heavier weight (CONFIG_PROVE_RCU is required), but
tells you in what function the offending RCU read-side critical section
was entered.

							Thanx, Paul