Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function
 v3
To:     Maarten Lankhorst <dev@mblankhorst.nl>,
        Peter Zijlstra <peterz@infradead.org>,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
References: <20180220125829.27060-1-christian.koenig@amd.com>
 <20180220131253.GF25314@hirez.programming.kicks-ass.net>
 <8fd80334-4d0e-8ed0-8a09-02a7e36a0eae@gmail.com>
 <20180220135709.GD25201@hirez.programming.kicks-ass.net>
 <b625cbca-c05a-afd1-fe97-a7a0760383c1@amd.com>
 <20180220145413.GF25201@hirez.programming.kicks-ass.net>
 <2dac8ec9-c54a-7dd1-af4e-62f2bc1a959c@gmail.com>
 <20180220152158.GH25201@hirez.programming.kicks-ass.net>
 <20180220235621.GD22199@phenom.ffwll.local>
 <9d782099-567f-38cb-391a-3c2a50cd5edd@mblankhorst.nl>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
Message-ID: <4db106d0-6df3-f7cf-2e03-44f346660a31@amd.com>
Date:   Wed, 21 Feb 2018 12:50:01 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.6.0
MIME-Version: 1.0
In-Reply-To: <9d782099-567f-38cb-391a-3c2a50cd5edd@mblankhorst.nl>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Received-SPF: None (protection.outlook.com: amd.com does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 Feb 2018 11:50:13.1794 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 7aff87d7-84f0-422c-7f70-08d579214545
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY4PR12MB1301
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.