linux-kernel - [PATCH 3/5] locking/lockdep: Add a faster path in __lock

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Message-Id: <1538157201-29173-4-git-send-email-longman@redhat.com>
Date:   Fri, 28 Sep 2018 13:53:19 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH 3/5] locking/lockdep: Add a faster path in __lock_release()

When __lock_release() is called, the most likely unlock scenario is
on the innermost lock in the chain.  In this case, we can skip some of
the checks and provide a faster path to completion.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/lockdep.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index add0468..ca002c0 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3625,6 +3625,13 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 	curr->lockdep_depth = i;
 	curr->curr_chain_key = hlock->prev_chain_key;
 
+	/*
+	 * The most likely case is when the unlock is on the innermost
+	 * lock. In this case, we are done!
+	 */
+	if (i == depth - 1)
+		return 1;
+
 	if (reacquire_held_locks(curr, depth, i + 1))
 		return 0;
 
@@ -3632,10 +3639,14 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
 	 * We had N bottles of beer on the wall, we drank one, but now
 	 * there's not N-1 bottles of beer left on the wall...
 	 */
-	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1))
-		return 0;
+	DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1);
 
-	return 1;
+	/*
+	 * Since reacquire_held_locks() would have called check_chain_key()
+	 * indirectly via __lock_acquire(), we don't need to do it again
+	 * on return.
+	 */
+	return 0;
 }
 
 static int __lock_is_held(const struct lockdep_map *lock, int read)
-- 
1.8.3.1