Messages in this thread Patch in this message | | | Date | Mon, 26 Jan 2009 09:12:34 -0800 | From | Darren Hart <> | Subject | Re: kernel BUG at fs/ext/super.c:428 |
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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@us.ibm.com>
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@us.ibm.com> ---
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
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