linux-kernel - Re: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a drm

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Message-ID: <a69a6870-18fe-5bbc-074e-871753a8f593@gmail.com>
Date:   Wed, 18 Aug 2021 22:50:00 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, sumit.semwal@...aro.org,
        christian.koenig@....com, axboe@...nel.dk, oleg@...hat.com,
        tglx@...utronix.de, dvyukov@...gle.com, walter-zh.wu@...iatek.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a
 drm_file's master

On 18/8/21 6:05 pm, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 03:38:17PM +0800, Desmond Cheong Zhi Xi wrote:
>> When drm_file.master changes value, the corresponding
>> drm_device.master_lookup_lock should be held.
>>
>> In drm_master_release, a call to drm_master_put sets the
>> file_priv->master to NULL, so we protect this section with
>> drm_device.master_lookup_lock.
>>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
> 
> At this points all refcounts to drm_file have disappeared, so yeah this is
> a lockless access, but also no one can observe it anymore. See also next
> patch.
> 
> Hence I think the current code is fine.
> -Daniel
> 

Ah ok got it, I didn't realize that. I'll drop patch 2 and 3 from the 
series then.

>> ---
>>   drivers/gpu/drm/drm_auth.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index 8efb58aa7d95..8c0e0dba1611 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -373,8 +373,11 @@ void drm_master_release(struct drm_file *file_priv)
>>   	}
>>   
>>   	/* drop the master reference held by the file priv */
>> -	if (file_priv->master)
>> +	if (file_priv->master) {
>> +		spin_lock(&dev->master_lookup_lock);
>>   		drm_master_put(&file_priv->master);
>> +		spin_unlock(&dev->master_lookup_lock);
>> +	}
>>   	mutex_unlock(&dev->master_mutex);
>>   }
>>   
>> -- 
>> 2.25.1
>>
>