[PATCH tip/core/rcu 7/9] rcu: Pretend ->boost_mtx acquired legitimately

[Paul E. McKenney]

RCU priority boosting uses rt_mutex_init_proxy_locked() to initialize an
rt_mutex structure in locked state held by some other task.  When that
other task releases it, lockdep complains (quite accurately, but a bit
uselessly) that the other task never acquired it.  This complaint can
suppress other, more helpful, lockdep complaints, and in any case it is
a false positive.

This commit therefore uses the mutex_acquire() macro to make it look
like that other process legitimately acquired the lock, thus suppressing
this lockdep false-positive complaint.

Of course, if lockdep ever learns about rt_mutex_init_proxy_locked(),
this commit will need to be reverted.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index fed95fa941e6..60bfb16c9a1a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -529,8 +529,11 @@ void rcu_read_unlock_special(struct task_struct *t)
 		}
 
 		/* Unboost if we were boosted. */
-		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
+		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex) {
+			/* For lockdep, pretend we acquired lock honestly. */
+			mutex_acquire(&rnp->boost_mtx.dep_map, 0, 0, _RET_IP_);
 			rt_mutex_unlock(&rnp->boost_mtx);
+		}
 
 		/*
 		 * If this was the last task on the expedited lists,
-- 
2.5.2