linux-kernel - Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned

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Message-ID: <4db106d0-6df3-f7cf-2e03-44f346660a31@amd.com>
Date:   Wed, 21 Feb 2018 12:50:01 +0100
From:   Christian König <christian.koenig@....com>
To:     Maarten Lankhorst <dev@...ankhorst.nl>,
        Peter Zijlstra <peterz@...radead.org>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function
 v3

Am 21.02.2018 um 11:54 schrieb Maarten Lankhorst:
> 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..

Yeah, but as I noted now multiple times that won't work.

See I need to distinct between the BOs acquired with and without a 
context. Otherwise the whole approach doesn't make much sense.

Christian.