linux-kernel - Re: [PATCH] drm/amdgpu: fix deadlock while reading mqd from debugfs

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Date: Tue, 26 Mar 2024 16:25:10 +0100
From: "Sharma, Shashank" <shashank.sharma@....com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Christian König <ckoenig.leichtzumerken@...il.com>,
 Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amdgpu: fix deadlock while reading mqd from debugfs

Thanks for the patch,

Patch pushed for staging.


Regards

Shashank

On 25/03/2024 00:23, Alex Deucher wrote:
> On Sat, Mar 23, 2024 at 4:47 PM Sharma, Shashank
> <shashank.sharma@....com> wrote:
>>
>> On 23/03/2024 15:52, Johannes Weiner wrote:
>>> On Thu, Mar 14, 2024 at 01:09:57PM -0400, Johannes Weiner wrote:
>>>> Hello,
>>>>
>>>> On Fri, Mar 08, 2024 at 12:32:33PM +0100, Christian König wrote:
>>>>> Am 07.03.24 um 23:07 schrieb Johannes Weiner:
>>>>>> Lastly I went with an open loop instead of a memcpy() as I wasn't
>>>>>> sure if that memory is safe to address a byte at at time.
>>>> Shashank pointed out to me in private that byte access would indeed be
>>>> safe. However, after actually trying it it won't work because memcpy()
>>>> doesn't play nice with mqd being volatile:
>>>>
>>>> /home/hannes/src/linux/linux/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c: In function 'amdgpu_debugfs_mqd_read':
>>>> /home/hannes/src/linux/linux/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c:550:22: warning: passing argument 1 of '__builtin_dynamic_object_size' discards 'volatil' qualifier from pointer target type [-Wdiscarded-qualifiers]
>>>>     550 |         memcpy(kbuf, mqd, ring->mqd_size);
>>>>
>>>> So I would propose leaving the patch as-is. Shashank, does that sound
>>>> good to you?
>>> Friendly ping :)
>>>
>>> Shashank, is your Reviewed-by still good for this patch, given the
>>> above?
>> Ah, sorry I missed this due to some parallel work, and just realized the
>> memcpy/volatile limitation.
>>
>> I also feel the need of protecting MQD read under a lock to avoid
>> parallel change in MQD while we do byte-by-byte copy, but I will add
>> that in my to-do list.
>>
>> Please feel free to use my R-b.
> Shashank, if the patch looks good, can you pick it up and apply it?
>
> Alex
>
>
>> - Shashank
>>
>>> Thanks