Re: [RFC][PATCH -v2 1/4] locking/drm/i915: Kill mutex trickery

[Waiman Long]

On 08/26/2016 05:10 AM, Peter Zijlstra wrote:
> On Fri, Aug 26, 2016 at 05:25:09AM +0200, Andreas Mohr wrote:
> > > Another alternative is to provide a standard mutex API that returns the
> > > owner of the lock if there is a real need for this capability. Peeking
> > > into lock internal is not a good practice.
>
> > So, it seems the most we could provide which would offer a reliable,
> > non-racy API protocol is something like:
> >
> > static bool mutex_is_locked_by_us(struct mutex *mutex)
> >
> > since during execution of this processing it would be guaranteed that:
> > - either we do have the lock, thus *we* *RELIABLY* are and will be "the owner"
> > - or we simply do not have it, thus *we* *RELIABLY* are and will be "not the owner"
> Right, and that is exactly what they attempted and need. And the new
> mutex implementation could actually do this much better than the old
> one.
>
> But yes, such an interface should be part of the mutex implementation
> proper, not something hacked on in random places.

It is what exactly I have in mind. The actual API implemented is subject 
to negotiation. The important thing is that it has to be within the core 
mutex code.

> Fwiw, the build bot seems to have found another instance of this thing
> :/ drivers/gpu/drm/msm/msm_gem_shrinker.c includes an exact copy.

This seems to be a new file that was introduced since 4.8.

Cheers,
Longman