[PATCH RT 07/22] rcu: Suppress lockdep false-positive ->boost_mtx complaints

[Julia Cartwright]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

4.9.115-rt94-rc1 stable review patch.
If you have any objection to the inclusion of this patch, let me know.

--- 8< --- 8< --- 8< ---
[ Upstream commit 02a7c234e54052101164368ff981bd72f7acdd65 ]

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 switches from rt_mutex_unlock() to
rt_mutex_futex_unlock(), thereby avoiding the lockdep annotations.
Of course, if lockdep ever learns about rt_mutex_init_proxy_locked(),
addtional adjustments will be required.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Julia Cartwright <julia@xxxxxx>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3d18d08e8382..510de72ad8a3 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -486,7 +486,7 @@ void rcu_read_unlock_special(struct task_struct *t)
 
 		/* Unboost if we were boosted. */
 		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
-			rt_mutex_unlock(&rnp->boost_mtx);
+			rt_mutex_futex_unlock(&rnp->boost_mtx);
 
 		/*
 		 * If this was the last task on the expedited lists,
-- 
2.18.0