linux-kernel - [PATCH 1/2 v2] lockdep: Fix redundant_hardirqs

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Date:	Thu, 15 Apr 2010 23:10:42 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	David Miller <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [PATCH 1/2 v2] lockdep: Fix redundant_hardirqs_on incremented with irqs enabled

When a path restore the flags while irqs are already enabled, we
update the per cpu var redundant_hardirqs_on in a racy fashion
and debug_atomic_inc() warns about this situation.

In this particular case, loosing a few hits in a stat is not a big
deal, so increment it without protection.

v2: Don't bother with disabling irq, we can miss one count in
    rare situations

Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: David Miller <davem@...emloft.net>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
---
 kernel/lockdep.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 78325f8..1b58a1b 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2298,7 +2298,12 @@ void trace_hardirqs_on_caller(unsigned long ip)
 		return;
 
 	if (unlikely(curr->hardirqs_enabled)) {
-		debug_atomic_inc(redundant_hardirqs_on);
+		/*
+		 * Neither irq nor preemption are disabled here
+		 * so this is racy by nature but loosing one hit
+		 * in a stat is not a big deal.
+		 */
+		this_cpu_inc(lockdep_stats.redundant_hardirqs_on);
 		return;
 	}
 	/* we'll do an OFF -> ON transition: */
-- 
1.6.2.3

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