linux-kernel - Re: [PATCH 1/4] lockdep: lock_set

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Date:	Tue, 8 Nov 2011 17:07:35 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	linux-kernel@...r.kernel.org, sergey.senozhatsky@...il.com,
	bp@...en8.de, Ingo Molnar <mingo@...e.hu>,
	Tejun Heo <tj@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	casteyde.christian@...e.fr
Subject: Re: [PATCH 1/4] lockdep: lock_set_subclass() fix

On Tue, Nov 08, 2011 at 09:46:20AM +0100, Peter Zijlstra wrote:
> On Tue, 2011-11-08 at 16:14 +0800, Yong Zhang wrote:
> > 
> > But how do we deal with ->class_cache? Always set it in
> > loop_up_lock_class()? 
> 
> Hrm.. good point, aside from that there's another problem as well, I
> think we can deal with the cache being NULL, but is memset() an atomic
> write? If not a read could observe an intermediate state and go funny.

Yup.

> 
> I'm tempted to go with the pure kmemcheck_mark_initialized() thing for
> now.

me too :)

So something like below?

Thanks,
Yong
---
From: Yong Zhang <yong.zhang0@...il.com>
Subject: [PATCH] lockdep: kmemcheck: annotate ->lock in lockdep_init_map()

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;

So annotate ->lock with kmemcheck_mark_initialized() to cure this problem
and the one reported in commit f59de89.

Reported-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Reported-by: Borislav Petkov <bp@...en8.de>
Suggested-by: Vegard Nossum <vegard.nossum@...il.com>
Signed-off-by: Yong Zhang <yong.zhang0@...il.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Tejun Heo <tj@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
---
 kernel/lockdep.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index e69434b..08a2b1b 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2948,7 +2948,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 void lockdep_init_map(struct lockdep_map *lock, const char *name,
 		      struct lock_class_key *key, int subclass)
 {
-	memset(lock, 0, sizeof(*lock));
+	kmemcheck_mark_initialized(lock, sizeof(*lock));
 
 #ifdef CONFIG_LOCK_STAT
 	lock->cpu = raw_smp_processor_id();
-- 
1.7.5.4

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