linux-kernel - Re: [PATCH v2 0/4] gpio: fix an incorrect lockdep warning

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Message-ID: <20160919090358.GU5016@twins.programming.kicks-ass.net>
Date:   Mon, 19 Sep 2016 11:03:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Vignesh R <vigneshr@...com>, Yong Li <yong.b.li@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Ingo Molnar <mingo@...hat.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] gpio: fix an incorrect lockdep warning

On Mon, Sep 19, 2016 at 10:48:44AM +0200, Peter Rosin wrote:
> On 2016-09-19 10:14, Peter Zijlstra wrote:
> > On Mon, Sep 19, 2016 at 10:01:49AM +0200, Peter Rosin wrote:
> >> Or, do what the i2c-mux code is doing and use an rt_mutex instead
> >> of an ordinary mutex. That way you are very sure to not get any
> >> lockdep splat ... at all. Ok, sorry, that was not a serious
> >> suggestion, but it would be a tad bit simpler to implement...
> > 
> > So I find it weird that people use rt_mutex as a locking primitive,
> > since its only that one lock that then does PI and all the other locks
> > that are related still create inversions.
> 
> So, someone took the bait :-)
> 
> Yes, I too find it weird, and would like to get rid of it. It's just
> odd. It's been some years since the start though, waaay before me
> entering kernel space.
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=194684e596af4b
> 
> 
> But it's hard to argue with the numbers given in the discussion:
> 
> http://linux-i2c.vger.kernel.narkive.com/nokldJcc/patch-1-1-i2c-prevent-priority-inversion-on-top-of-bus-lock
> 
> Has anything happened to the regular mutex implementation that might
> have changed the picture? *crosses fingers*

Use the -RT kernel and all locks will end up as rt_mutex. Avoiding
inversion on one specific lock, while there are then a gazillion other
than can equally create inversion doesn't make sense to me.