Re: WARNING: at kernel/lockdep.c:690 __lock_acquire+0x168/0x164b()

[Yong Zhang]

On Fri, Oct 21, 2011 at 02:14:34AM -0700, David Rientjes wrote:
> How does it mask the race condition?  Before the memset(), the ->name 
> field was never _cleared_ in lockdep_init_map() like it is now, it was 
> only stored.

A typcal race condition will like this:

	CPU A					CPU B
lock_set_subclass(lockA);
  lock_set_class(lockA);
    lockdep_init_map(lockA);
      /* lockA->name is cleared */
      memset(lockA);
					__lock_acquire(lockA);
					  /* lockA->class_cache[] is cleared */
					  register_lock_class(lockA);
					    look_up_lock_class(lockA);
					      WARN_ON_ONCE(class->name !=
							lock->name);

      lock->name = name;

And a untested patch is below:
BTW, now the patch could cure (I guess) the very issue reported
in this thread.
But it don't cover the case which change the key and the relevant
lock_class has existed, I don't think out a way how to fix it yet :)
But the fact is we have no such caller yet, the only call site of
lock_set_subclass() is double_unlock_balance().

Thanks,
Yong

---
From: Yong Zhang <yong.zhang0@xxxxxxxxx>
Subject: [PATCH] lockdep: On-demand initialization for lock_set_class()

Since commit f59de89 [lockdep: Clear whole lockdep_map on initialization],
lockdep_init_map() will clear all the struct. But it will break
lock_set_class()/lock_set_subclass(). A typical race condition
is like below:

	CPU A					CPU B
lock_set_subclass(lockA);
  lock_set_class(lockA);
    lockdep_init_map(lockA);
      /* lockA->name is cleared */
      memset(lockA);
					__lock_acquire(lockA);
					  /* lockA->class_cache[] is cleared */
					  register_lock_class(lockA);
					    look_up_lock_class(lockA);
					      WARN_ON_ONCE(class->name !=
							lock->name);

      lock->name = name;

Reported-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Reported-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/lockdep.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 91d67ce..bc7dd1e 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3160,7 +3160,10 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 	return print_unlock_inbalance_bug(curr, lock, ip);
 
 found_it:
-	lockdep_init_map(lock, name, key, 0);
+	/* only changing lock->name make no sense */
+	WARN_ON(lock->key == key && lock->name != name);
+	if (lock->key != key)
+		lockdep_init_map(lock, name, key, 0);
 	class = register_lock_class(lock, subclass, 0);
 	hlock->class_idx = class - lock_classes + 1;
 
-- 
1.7.5.4


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