From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/2] remove recursive call to migrate_disable in
 read_lock_bh
Date: Thu, 21 Nov 2013 11:22:13 +0100
Message-ID: <20131121102213.GA10022@twins.programming.kicks-ass.net>
References: <20131120102107.GA12022@opentech.at>
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Cc: linux-rt-users@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Andreas Platschek <platschek@ict.tuwien.ac.at>
To: Nicholas Mc Guire <der.herr@hofr.at>
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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.