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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@redhat.com>
AuthorDate: Mon, 20 Jan 2014 19:20:10 +0100
Committer: Ingo Molnar <mingo@kernel.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@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140120182010.GA26498@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.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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