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Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function
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To:     Peter Zijlstra <peterz@infradead.org>, christian.koenig@amd.com,
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From:   Maarten Lankhorst <dev@mblankhorst.nl>
Message-ID: <9d782099-567f-38cb-391a-3c2a50cd5edd@mblankhorst.nl>
Date:   Wed, 21 Feb 2018 11:54:11 +0100
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Op 21-02-18 om 00:56 schreef Daniel Vetter:
> On Tue, Feb 20, 2018 at 04:21:58PM +0100, Peter Zijlstra wrote:
>> On Tue, Feb 20, 2018 at 04:05:49PM +0100, Christian König wrote:
>>> Am 20.02.2018 um 15:54 schrieb Peter Zijlstra:
>>>> On Tue, Feb 20, 2018 at 03:34:07PM +0100, Christian König wrote:
>>>>>> OK, but neither case would in fact need the !ctx case right? That's just
>>>>>> there for completeness sake?
>>>>> Unfortunately not. TTM uses trylock to lock BOs which are about to be
>>>>> evicted to make room for all the BOs locked with a ctx.
>>>>>
>>>>> I need to be able to distinct between the BOs which are trylocked and those
>>>>> which are locked with a ctx.
>>>>>
>>>>> Writing this I actually noticed the current version is buggy, cause even
>>>>> when we check the mutex owner we still need to make sure that the ctx in the
>>>>> lock is NULL.
>>>> Hurm... I can't remember why trylocks behave like that, and it seems
>>>> rather unfortunate / inconsistent.
>>> Actually for me that is rather fortunate, cause I need to distinct between
>>> the locks acquired through trylock and lock.
>> I suppose that would always be possible using:
>> ww_mutex_trylock(.ctx=NULL), and it could be that there simply weren't
>> any immediate uses for a !NULL trylock and it was thus not implemented.
>>
>> But that is all very long ago..
> I think we simple never had a use-case for interleaving ww_mutex_lock(ctx)
> and ww_mutex_trylock(ctx). Nesting multiple trylocks in ctx-locks happens
> plenty, but not further:
>
> The common use-case for that is locking a bunch of buffers you need (for
> command submission or whatever), and then trylocking other buffers to make
> space for the buffers you need to move into VRAM. I guess if only
> trylocking buffers doesn't succeed in freeing up enough VRAM then we could
> go into blocking ww_mutex_locks which need the ctx (and which would need
> all the trylock-acquired buffers to be annotated with the ctx too). TTM
> currently tries to be far enough away from that corner case (using a
> defensive "never use more than 50% of all memory for gfx" approach) that
> it doesn't seem to need that.
>
> Once we get there it should indeed be simply to add a ctx parameter to
> ww_mutex_trylock to fix this case. The TTM side rework is definitely going
> to be the much bigger issue here ...
> -Daniel

Yes, I think fixing trylock to take a ctx parameter would be a better fix than ww_mutex_is_owned_by..