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.
well that was what I ment with the nned to understand the interleavings of
better - playing with a cocci patch to catch these "equivalent" call sequences
unfortunately it did not trigger - so I promply concluded that all is
fine...
thx!
hofrat
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