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

From: Peter Rosin
Date: Mon Sep 19 2016 - 04:50:11 EST

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.

But it's hard to argue with the numbers given in the discussion:

Has anything happened to the regular mutex implementation that might
have changed the picture? *crosses fingers*

> In any case, since people have started doing this, adding lockdep
> support for rt_mutex is on the todo _somewhere_, so don't expect that to
> avoid splats forever.

I was actually looking quite hard to find out how I should declare the
lockdep class for the rt_mutex in order to prevent future splats, before
I realized that it wasn't even possible...