linux-kernel - [tip:core/locking] lockdep: Don' t create the wrong dependency on hlock->check == 0

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Date:	Mon, 10 Feb 2014 05:33:08 -0800
From:	tip-bot for Oleg Nesterov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, sasha.levin@...cle.com,
	hpa@...or.com, mingo@...nel.org, stern@...land.harvard.edu,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	davej@...hat.com, gregkh@...uxfoundation.org,
	paulmck@...ux.vnet.ibm.com, rostedt@...dmis.org,
	tglx@...utronix.de, oleg@...hat.com
Subject: [tip:core/locking] lockdep: Don'
 t create the wrong dependency on hlock->check == 0

Commit-ID:  1b5ff816cab708ba44c7d7b56b613516269eb577
Gitweb:     http://git.kernel.org/tip/1b5ff816cab708ba44c7d7b56b613516269eb577
Author:     Oleg Nesterov <oleg@...hat.com>
AuthorDate: Mon, 20 Jan 2014 19:20:10 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 9 Feb 2014 21:18:57 +0100

lockdep: Don't create the wrong dependency on hlock->check == 0

Test-case:

	DEFINE_MUTEX(m1);
	DEFINE_MUTEX(m2);
	DEFINE_MUTEX(mx);

	void lockdep_should_complain(void)
	{
		lockdep_set_novalidate_class(&mx);

		// m1 -> mx -> m2
		mutex_lock(&m1);
		mutex_lock(&mx);
		mutex_lock(&m2);
		mutex_unlock(&m2);
		mutex_unlock(&mx);
		mutex_unlock(&m1);

		// m2 -> m1 ; should trigger the warning
		mutex_lock(&m2);
		mutex_lock(&m1);
		mutex_unlock(&m1);
		mutex_unlock(&m2);
	}

this doesn't trigger any warning, lockdep can't detect the trivial
deadlock.

This is because lock(&mx) correctly avoids m1 -> mx dependency, it
skips validate_chain() due to mx->check == 0. But lock(&m2) wrongly
adds mx -> m2 and thus m1 -> m2 is not created.

rcu_lock_acquire()->lock_acquire(check => 0) is fine due to read == 2,
so currently only __lockdep_no_validate__ can trigger this problem.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Cc: Dave Jones <davej@...hat.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Sasha Levin <sasha.levin@...cle.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20140120182010.GA26498@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.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 8c85a0d..f7eba92 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1936,12 +1936,12 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
 
 	for (;;) {
 		int distance = curr->lockdep_depth - depth + 1;
-		hlock = curr->held_locks + depth-1;
+		hlock = curr->held_locks + depth - 1;
 		/*
 		 * Only non-recursive-read entries get new dependencies
 		 * added:
 		 */
-		if (hlock->read != 2) {
+		if (hlock->read != 2 && hlock->check) {
 			if (!check_prev_add(curr, hlock, next,
 						distance, trylock_loop))
 				return 0;
--
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