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Date: Mon, 13 Feb 2017 12:20:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Andy Ritger <aritger@nvidia.com>, Alex Goins <agoins@nvidia.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, linux-rt-users@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH RT] Align rt_mutex inlining with upstream behavior
Message-ID: <20170213112052.GA6557@twins.programming.kicks-ass.net>
References: <1485312350-1265-1-git-send-email-agoins@nvidia.com>
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On Fri, Feb 10, 2017 at 06:50:50PM +0100, Sebastian Andrzej Siewior wrote:
> Alex Goins reported that mutex_destroy() on RT will force a GPL only symbol
> which won't link and therefore fail on a non-GPL kernel module.
> This does not happen on !RT and is a regression on RT which we would like to
> avoid.
> I try here the easy thing and to not use rt_mutex_destroy() if
> CONFIG_DEBUG_MUTEXES is not enabled. This will still break for the DEBUG
> configs so instead of adding a wrapper around rt_mutex_destroy() (which we have
> for rt_mutex_lock() for instance) I am simply dropping the GPL part from the
> export.
> 
> Reported-by: Alex Goins <agoins@nvidia.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/mutex_rt.h |    5 +++++
>  kernel/locking/rtmutex.c |    3 +--
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/mutex_rt.h
> +++ b/include/linux/mutex_rt.h
> @@ -43,7 +43,12 @@ extern void __lockfunc _mutex_unlock(str
>  #define mutex_lock_killable(l)		_mutex_lock_killable(l)
>  #define mutex_trylock(l)		_mutex_trylock(l)
>  #define mutex_unlock(l)			_mutex_unlock(l)
> +
> +#ifdef CONFIG_DEBUG_MUTEXES
>  #define mutex_destroy(l)		rt_mutex_destroy(&(l)->lock)
> +#else
> +static inline void mutex_destroy(struct mutex *lock) {}
> +#endif
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  # define mutex_lock_nested(l, s)	_mutex_lock_nested(l, s)
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -2027,8 +2027,7 @@ void rt_mutex_destroy(struct rt_mutex *l
>  	lock->magic = NULL;
>  #endif
>  }
> -
> -EXPORT_SYMBOL_GPL(rt_mutex_destroy);
> +EXPORT_SYMBOL(rt_mutex_destroy);

I don't understand:

$ git grep "EXPORT_SYMBOL.*mutex_destroy"
kernel/locking/mutex-debug.c:EXPORT_SYMBOL_GPL(mutex_destroy);
kernel/locking/rtmutex.c:EXPORT_SYMBOL_GPL(rt_mutex_destroy);

Your proposed patch makes it different from mainline.