[PATCH v2 tip/core/rcu 04/39] srcu: Check for tardy grace-period activity in cleanup_srcu_struct()

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

Users of SRCU are obliged to complete all grace-period activity before
invoking cleanup_srcu_struct().  This means that all calls to either
synchronize_srcu() or synchronize_srcu_expedited() must have returned,
and all calls to call_srcu() must have returned, and the last call to
call_srcu() must have been followed by a call to srcu_barrier().
Furthermore, the caller must have done something to prevent any
further calls to synchronize_srcu(), synchronize_srcu_expedited(),
and call_srcu().

Therefore, if there has ever been an invocation of call_srcu() on
the srcu_struct in question, the sequence of events must be as
follows:

1.  Prevent any further calls to call_srcu().
2.  Wait for any pre-existing call_srcu() invocations to return.
3.  Invoke srcu_barrier().
4.  It is now safe to invoke cleanup_srcu_struct().

On the other hand, if there has ever been a call to synchronize_srcu()
or synchronize_srcu_expedited(), the sequence of events must be as
follows:

1.  Prevent any further calls to synchronize_srcu() or
    synchronize_srcu_expedited().
2.  Wait for any pre-existing synchronize_srcu() or
    synchronize_srcu_expedited() invocations to return.
3.  It is now safe to invoke cleanup_srcu_struct().

If there have been calls to all both types of functions (call_srcu()
and either of synchronize_srcu() and synchronize_srcu_expedited()), then
the caller must do the first three steps of the call_srcu() procedure
above and the first two steps of the synchronize_s*() procedure above,
and only then invoke cleanup_srcu_struct().

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/srcu.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index ba41a5d04b49..6beeba7b0b67 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -261,6 +261,11 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 {
 	if (WARN_ON(srcu_readers_active(sp)))
 		return; /* Leakage unless caller handles error. */
+	if (WARN_ON(!rcu_all_batches_empty(sp)))
+		return; /* Leakage unless caller handles error. */
+	flush_delayed_work(&sp->work);
+	if (WARN_ON(sp->running))
+		return; /* Caller forgot to stop doing call_srcu()? */
 	free_percpu(sp->per_cpu_ref);
 	sp->per_cpu_ref = NULL;
 }
-- 
2.5.2