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

Paul E. McKenney
2011-06-07T00:42:54Z

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