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

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Message-ID: <57C05333.8010900@hpe.com>
Date:   Fri, 26 Aug 2016 10:33:23 -0400
From:   Waiman Long <waiman.long@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Andreas Mohr <andi@...as.de>,
        Daniel Vetter <daniel.vetter@...el.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH -v2 1/4] locking/drm/i915: Kill mutex trickery

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