linux-kernel - Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu

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Message-ID: <9c0175bd-cd5e-e643-de9b-5b2f6b2449a1@gmail.com>
Date:   Tue, 20 Feb 2018 13:31:14 +0100
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     christian.koenig@....com, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu

Am 20.02.2018 um 12:33 schrieb Daniel Vetter:
> [SNIP]
> Ah, so the ttm_ctx I've spotted was something entirely different and
> doesn't contain the ww_acquire_ctx (I didn't check)? I'd assume you have
> the same ctx passed around to everything in ttm, but if that doesn't exist
> then we can indeed not annotate ww_mutex_trylock_ctx with the right ctx.

Yes, exactly.

I actually tried this approach, e.g. put the ww_acquire_context into the 
ttm_operation_context and then use that with ww_mutex_trylock_ctx.

But a) that turned out to be to much hassle, e.g. at least amdgpu 
doesn't use a ww_acquire context in most cases.

And b) it actually wasn't what I was looking for, e.g. I couldn't 
distinct between the trylocked BOs an everything else any more.

>> [SNIP]
>> But to me it actually looks more like that makes it unnecessary complicated.
>> The use case in amdgpu which could only check the context isn't performance
>> critical.
> Oh I'm not worried about the runtime overhead at all, I'm worried about
> conceptual clarity of this stuff. If you have a ctx there's no need to
> also look at ->owner.
>
> Another idea: We drop the task argument from functions and go with the
> following logic:
>
> ww_mutex_is_owner(lock, ctx)
> {
> 	if (ctx)
> 		return lock->ctx == ctx;
> 	else
> 		return lock->owner == current;
> }
>
> I think that would solve your use case, and gives us the neat interface
> I'm aiming for. Kerneldoc can then explain what's happening for a NULL
> ctx.

Good point, going to adjust the patches this way and resend.

Christian.