Re: [Question] lockdep: Is nested lock handled correctly?

[Boqun Feng <boqun.feng@gmail.com>]

Hi Peter,

On Mon, Aug 10, 2015 at 04:24:17PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 10, 2015 at 09:49:24PM +0800, Boqun Feng wrote:

<snip>

> > Though I don't want to have a locking order like that either, we can't
> > stop others from using that order(maybe a good design review will) and
> > lockdep yells something -unrelated- in such an order.
> > 
> > I think we can either let lockdep complain if some one uses this
> > locking order or clean up current code a little bit to tolarent this.
> > 
> > If you really think we should do something about it, I can write the
> > patch and add test cases.
> 
> 
> Maybe something like the below in __lock_acquire():
> 
> 	/* Daft bugger, can't guard a nesting order with the same lock class */
> 	if (DEBUG_LOCKS_WARN_ON(lock == nest_lock))
> 		return 0;
> 
> ?

I may not understand this well.. but I think this may not detect the
problem. The problem is:

A correct nesting order get disturbed by other locks acquired before the
nested lock acquired and release before the nested, which makes two
held_lock structures merged during __lock_acquire().


I think we can detect this in __lock_release():

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 8acfbf7..e75f622 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3427,6 +3427,19 @@ found_it:
        curr->lockdep_depth = i;
        curr->curr_chain_key = hlock->prev_chain_key;
 
+       /*
+        * We are going to "reacquire" the rest of stack, but we find out
+        * __lock_acquire() will merge the next hlock into prev_hlock,
+        * which means this is not a good time to release this lock and lock
+        * users might need to reconsider the locking design.
+        */
+       if (prev_hlock && (i+1) < depth) {
+               hlock = curr->held_locks + i + 1;
+               if (DEBUG_LOCKS_WARN_ON(hlock->nest_lock &&
+                               hlock->class_idx == prev_hlock->class_idx))
+                       return 0;
+       }
+
        for (i++; i < depth; i++) {
                hlock = curr->held_locks + i;
                if (!__lock_acquire(hlock->instance,


Regards,
Boqun