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

Frederic Weisbecker
2011-06-07T00:19:15Z

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.

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, might_sleep() checks rcu_preempt_depth().