Re: kernel BUG at fs/ext/super.c:428

[Darren Hart]

Peter Zijlstra wrote:
> On Mon, 2009-01-26 at 08:39 -0800, Darren Hart wrote:
>
> > > diff --git a/kernel/futex.c b/kernel/futex.c
> > > index f89d373..f4132ab 100644
> > > --- a/kernel/futex.c
> > > +++ b/kernel/futex.c
> > > @@ -929,7 +929,7 @@ out_unlock:
> > >
> > >  	/* drop_futex_key_refs() must be called outside the spinlocks. */
> > >  	while (--drop_count >= 0)
> > > -		drop_futex_key_refs(&key1);
> > > +		drop_futex_key_refs(&key2);
> > Unfortunately, I realized later that this code was indeed correct and I 
> > asked Ingo to pull my patch implementing the above change.  Quoting my 
> > previous mail on the subject:
> >
> > "I believe what is happening here is that the requeue loop requeues each 
> > waiter from one futex (key1) to another (key2).  It rightly takes a 
> > reference to the futex at key2 and then decrements the references to 
> > key1 by drop_count (since the waiters now reference key2, not key1). 
> > The newly taken key2 references will be dropped in futex_wait() when 
> > each waiter is woken up and takes the futex."
>
> Argh, that wants a comment..

Agreed.  How about this.


futex: comment requeue key reference symmantics

From: Darren Hart <dvhltc@xxxxxxxxxx>

We've tripped over the futex_requeue drop_count refering to key2
instead of key1.  The code is actually correct, but is non-intuitive.
This patch adds an explicit comment explaining the requeue.

Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
---

kernel/futex.c |    7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)


diff --git a/kernel/futex.c b/kernel/futex.c
index 8af1002..3655cad 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -969,7 +969,12 @@ out_unlock:
	if (hb1 != hb2)
		spin_unlock(&hb2->lock);

-	/* drop_futex_key_refs() must be called outside the spinlocks. */
+	/*
+	 * drop_futex_key_refs() must be called outside the spinlocks. During
+	 * the requeue we moved futex_q's from the hash bucket at key1 to the
+	 * one at key2 and updated their key pointer.  We no longer need to
+	 * hold the references to key1.
+	 */
	while (--drop_count >= 0)
		drop_futex_key_refs(&key1);



--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/