[RFC] Catch dwmw2's deadlock

[Matthew Wilcox <willy@infradead.org>]

<dwmw2> why doesn't lockdep catch us calling synchronize_srcu() with
   a lock held, and elsewhere obtaining that lock within an srcu critical
   region ("read lock") ?

Because synchronize_srcu() doesn't acquire the lock, merely checks that
it isn't held.

Would this work?  Not even compile tested.

You can put my SoB on this if it works.

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index ca4b5dcec675..e9c2ab8369c0 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1267,11 +1267,11 @@ static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
 {
 	struct rcu_synchronize rcu;
 
-	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
-			 lock_is_held(&rcu_bh_lock_map) ||
+	rcu_lock_acquire(&ssp->dep_map);
+	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
 			 lock_is_held(&rcu_lock_map) ||
 			 lock_is_held(&rcu_sched_lock_map),
-			 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
+			 "Illegal synchronize_srcu() in RCU read-side critical section");
 
 	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
 		return;
@@ -1282,6 +1282,7 @@ static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
 	__call_srcu(ssp, &rcu.head, wakeme_after_rcu, do_norm);
 	wait_for_completion(&rcu.completion);
 	destroy_rcu_head_on_stack(&rcu.head);
+	rcu_lock_release(&ssp->dep_map);
 
 	/*
 	 * Make sure that later code is ordered after the SRCU grace