Re: [PATCH 1/2] remove recursive call to migrate_disable in read_lock_bh

[Nicholas Mc Guire <der.herr@hofr.at>]

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@hofr.at>
> > 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@hofr.at>
> > ---
> >  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@infradead.org
> 
> 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