linux-kernel - Re: [PATCH RT] Align rt_mutex inlining with upstream behavior

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Date:   Mon, 30 Jan 2017 09:35:34 -0800
From:   Andy Ritger <aritger@...dia.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:     Alex Goins <agoins@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH RT] Align rt_mutex inlining with upstream behavior

On Thu, Jan 26, 2017 at 06:01:09PM +0100, Sebastian Andrzej Siewior wrote:
> On 2017-01-24 18:45:50 [-0800], Alex Goins wrote:
> > mutex_destroy is no-op inline when DEBUG_MUTEX is not enabled. The RT Linux
> > patches replace mutex_destroy() with rt_mutex_destroy(). This patch aligns
> > rt_mutex_destroy() with mutex_destroy() by using the same no-op inline
> > technique.
> > 
> > Signed-off-by: Alex Goins <agoins@...dia.com>
> > Reviewed-by: Andy Ritger <aritger@...dia.com>
> 
> So what is the problem? Why are we doing this? There is still a check to
> see if the lock is in use which is also done for the case where
> DEBUG_MUTEX is disabled.

The problem is that various static inline functions such as
reservation_object_fini() indirectly call mutex_destroy.  On DEBUG_MUTEX
kernels, mutex_destroy is EXPORT_SYMBOL_GPL.

In upstream, non-DEBUG_MUTEX kernels define mutex_destroy to a noop.
This gives users the option of disabling DEBUG_MUTEX if they want to
use non-GPL, reservation_object_fini()-using, kernel modules.

In PREEMPTRT, non-DEBUG_MUTEX kernels export rt_mutex_destroy as
EXPORT_SYMBOL_GPL, so users no longer have the work around of using
DEBUG_MUTEX.

This patch gives PREEMPTRT users the option of disabling DEBUG_MUTEX if
they want to use such kernel modules, matching upstream behavior.