linux-kernel - [PATCH 10/37] locking/lockdep: Make mark_lock() fastpath to work with multiple usage at once

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Date:   Thu, 28 Feb 2019 18:12:15 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 10/37] locking/lockdep: Make mark_lock() fastpath to work with multiple usage at once

Now that mark_lock() is going to handle multiple softirq vectors at
once for a given lock usage, the current fast path optimization that
simply check if the new bit usage is already present won't work anymore.

Indeed if the new usage is only partially present, such as for some
softirq vectors and not for others, we may spuriously ignore all the
verifications for the new vectors.

What we must check instead is a bit different: we have to make sure that
the new usage with all its vectors are entirely present in the current
usage mask before ignoring further checks.

Reviewed-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Pavan Kondeti <pkondeti@...eaurora.org>
Cc: Paul E . McKenney <paulmck@...ux.vnet.ibm.com>
Cc: David S . Miller <davem@...emloft.net>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/locking/lockdep.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 0988de06a7ed..9a5f2dbc3812 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3159,7 +3159,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	 * If already set then do not dirty the cacheline,
 	 * nor do any checks:
 	 */
-	if (likely(hlock_class(this)->usage_mask & new_mask))
+	if (likely(!(new_mask & ~hlock_class(this)->usage_mask)))
 		return 1;
 
 	if (!graph_lock())
@@ -3167,7 +3167,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	/*
 	 * Make sure we didn't race:
 	 */
-	if (unlikely(hlock_class(this)->usage_mask & new_mask)) {
+	if (unlikely(!(new_mask & ~hlock_class(this)->usage_mask))) {
 		graph_unlock();
 		return 1;
 	}
-- 
2.21.0