linux-kernel - [RFC/PATCH PATCH 1/6] lockdep: Remove redundant read checks.

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Message-ID: <20090511123930.31159.58092.stgit@sofia.in.ibm.com>
Date:	Mon, 11 May 2009 18:09:30 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul E McKenney <paulmck@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>
Cc:	linux-kernel@...r.kernel.org
Subject: [RFC/PATCH PATCH 1/6] lockdep: Remove redundant read checks.

1) In kernel/lockdep.c::validate_chain()
	ret = check_deadlock(curr, hlock, lock, hlock->read);
ret = 2 only if hlock->read = 2.

Hence,
	if (ret == 2)
		hlock->read = 2;
is redundant, hence can be removed.

2) In kernel/lockdep.c::check_prevs_add(curr, next)
	if (hlock->read != 2)
		check_prev_add(curr, hlock, next, distance);

Thus, check_prev_add is called only when hlock->read != 2.

>>From the conclusions of 2),
kernel/lockdep.c::check_prev_add(curr, prev, next, distance) gets called
iff prev->read != 2.
Hence, in kernel/lockdep.c::check_prev_add(curr, prev, next, distance)
	if (prev->read == 2)
		return 1;
is redunant, hence can be removed.


Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
---

 kernel/lockdep.c |    9 +--------
 1 files changed, 1 insertions(+), 8 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index accb40c..8d3c2b5 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1496,7 +1496,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 	 * write-lock never takes any other locks, then the reads are
 	 * equivalent to a NOP.
 	 */
-	if (next->read == 2 || prev->read == 2)
+	if (next->read == 2)
 		return 1;
 	/*
 	 * Is the <prev> -> <next> dependency already present?
@@ -1754,13 +1754,6 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
 		if (!ret)
 			return 0;
 		/*
-		 * Mark recursive read, as we jump over it when
-		 * building dependencies (just like we jump over
-		 * trylock entries):
-		 */
-		if (ret == 2)
-			hlock->read = 2;
-		/*
 		 * Add dependency only if this lock is not the head
 		 * of the chain, and if it's not a secondary read-lock:
 		 */

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