On Thu, 21 Nov 2013, Peter Zijlstra wrote:
> On Wed, Nov 20, 2013 at 11:21:07AM +0100, Nicholas Mc Guire wrote:
> > From 46393dc3185026c8500c2b734747d7c8785f3dc9 Mon Sep 17 00:00:00 2001
> > From: Nicholas Mc Guire <der.herr@xxxxxxx>
> > Date: Tue, 19 Nov 2013 23:31:05 -0500
> > Subject: [PATCH 1/2] remove recursive call to migrate_disable in read_lock_bh
> >
> > read_lock_bh/read_unlock_bh unconditionally calls local_bh_disable/enable
> > which already does a migrate_disable/enable - no need for this recursive call.
> >
> > patch is on top of 3.12-rt2
> >
> > No change of functionality
> >
> > Signed-off-by: Nicholas Mc Guire <der.herr@xxxxxxx>
> > ---
> > include/linux/rwlock_rt.h | 2 --
> > 1 files changed, 0 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/rwlock_rt.h b/include/linux/rwlock_rt.h
> > index 853ee36..87f5a1d 100644
> > --- a/include/linux/rwlock_rt.h
> > +++ b/include/linux/rwlock_rt.h
> > @@ -53,7 +53,6 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
> > #define read_lock_bh(lock) \
> > do { \
> > local_bh_disable(); \
> > - migrate_disable(); \
> > rt_read_lock(lock); \
> > } while (0)
> >
> > @@ -83,7 +82,6 @@ extern void __rt_rwlock_init(rwlock_t *rwlock, char *name, struct lock_class_key
> > #define read_unlock_bh(lock) \
> > do { \
> > rt_read_unlock(lock); \
> > - migrate_enable(); \
> > local_bh_enable(); \
> > } while (0)
>
> So the problem with this patch and the next is that:
>
> read_lock_bh();
>
> read_unlock();
> local_bh_enable();
>
> Is a valid pattern; and you'll notice that the release part has 2
> migrate put refs. So if you can make a patch similar to:
>
> lkml.kernel.org/r/20131120162736.624493595@xxxxxxxxxxxxx
>
> That allows read_lock_bh() to obtain 2 migrate disable refs in one go,
> then it would all work out just fine.
yup thats what was causing the oops - fixed by the patch below
with this my two boxes here survived cyclictest over night
showing the same results as without the recursive calls to
migrate_disable/enable removed.
>From 2c8e669b691b825c0ed2a02bd7a698d8ed5c6d29 Mon Sep 17 00:00:00 2001
From: Nicholas Mc Guire <der.herr@xxxxxxx>
Date: Thu, 21 Nov 2013 18:22:55 -0500
Subject: [PATCH] rebalance locks by converting write_lock_bh to write_lock+local_bh_disable
This patch just rebalances the lock api
in __neigh_event_send write_lock_bh(&neigh->lock) is implicitly balanced by
write_unlock(&neigh->lock)+local_bh_disable() - while this is equivalent with
respect to the effective low level locking primitives it breaks balancing
in the locking api. This makes automatic lock-checking trigger false
positives, creates an implicit dependency between *_lock_bh and *_lock
functions as well as making the extremly simply locking of net core even
easier to understand.
No change of functionality
Signed-off-by: Nicholas Mc Guire <der.herr@xxxxxxx>
---
net/core/neighbour.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index ca15f32..d681c75 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -966,7 +966,8 @@ int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
int rc;
bool immediate_probe = false;
- write_lock_bh(&neigh->lock);
+ local_bh_disable();
+ write_lock(&neigh->lock);
rc = 0;
if (neigh->nud_state & (NUD_CONNECTED | NUD_DELAY | NUD_PROBE))
--
1.7.2.5
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