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    SubjectRe: [1/4] rcu: Detect uses of rcu read side in extended quiescent states
    On Mon, Jun 06, 2011 at 11:10:21AM -0700, Paul E. McKenney wrote:
    > commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9
    > Author: Paul E. McKenney <>
    > 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 <>

    A little comment about this patch:

    > 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().

     \ /
      Last update: 2011-06-07 02:21    [W:0.023 / U:4.460 seconds]
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