Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <3da7b3c2-737e-a067-9018-8e743e59ef65@amd.com>
Date:   Mon, 5 Sep 2022 20:26:40 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH 1/4] dma-buf: Check status of enable-signaling bit on
 debug
Content-Language: en-US
To:     Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        Arvind Yadav <Arvind.Yadav@amd.com>, andrey.grodzovsky@amd.com,
        shashank.sharma@amd.com, amaranath.somalapuram@amd.com,
        Arunpravin.PaneerSelvam@amd.com, sumit.semwal@linaro.org,
        gustavo@padovan.org, airlied@linux.ie, daniel@ffwll.ch,
        linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org
References: <20220905105653.13670-1-Arvind.Yadav@amd.com>
 <20220905105653.13670-2-Arvind.Yadav@amd.com>
 <0038fcff-35f1-87e3-aa26-cdd104a13628@amd.com>
 <3c702549-75f4-c640-9f9c-37d7fcbb1645@linux.intel.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
In-Reply-To: <3c702549-75f4-c640-9f9c-37d7fcbb1645@linux.intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?L3F4SjdmQ2NpVDZSSXFGR2o3SHkvcDNCclZYa1NFSVJnbWRwVWR5MWZ2K29x?=
 =?utf-8?B?alNkUHBJQmY1ME5DRFdnSDdzdmIvMldCalNyQnduMFBpWTJVdDEzbms1MXVN?=
 =?utf-8?B?SWQzY002RzZXb3FoZXRKeDVKTFNzbWhsdzRLNExTOTVNN2VWZUM1V2xRRU0y?=
 =?utf-8?B?NkJnUklYQmpZdUE1K3lJYkNlRTl3SzcyRWhxZ1lEOGVwSDZpdjBzNWVpSGky?=
 =?utf-8?B?TlVZeE0zTGppRFNvNHJzTERHVkVRMERZTk9aWWRobDM4M1BuWXN5UU52ei9T?=
 =?utf-8?B?WkpaTkF0S2pBMWd5VjRsYWV1S2JJV0Z3N1N6YUZzUXNmbjNoTU1BSnRXZTdr?=
 =?utf-8?B?ZW11cno2U21rVHlrQUVnUStSclVneWh4TmV3VmNqY1dnb1NmMXlSSkZ5RkFq?=
 =?utf-8?B?SzJrSjZMeE5NYWxSa0l2RXl0YlBwbWZMcU5xcHRaNEVyUEtJVDJWaWZwcWNP?=
 =?utf-8?B?Ti9mZzU5L04vNTNtM0R0TWJVTTdkRnB5SnR0enhIaW03T0FLTlBhdDNaRHUv?=
 =?utf-8?B?d3g5NmJkTUJFS3BDZWRIOUlnQ0hhU3E5QjkyUWdnWkFUUU5udW5HcWxiemJU?=
 =?utf-8?B?VkxIRUgxcmdpYXRodGsrL2R3N0ZBcHpUdzVjMWZheThDV3h4Ylg1N1oyNHV2?=
 =?utf-8?B?NDJVdVFuaWd1bTRsVXRUcFYzUWhlS2pCQ2FwNjd0aXRheFBiVndvMjczWjcv?=
 =?utf-8?B?enBIRHpwRlQ0WlpySTlBSk55bGNvUncydTBnSy9IY0FyQkhzTkNiNzZPSGpO?=
 =?utf-8?B?WWtJdDhqLzVEdHNieTZlVkNyUThyWGdOeWZBZithek5vR1BoOFNqWVVkUDdw?=
 =?utf-8?B?WFAvOEhrZm9nN1dWaFZxL0tVMUJlemllYlRIcWdmK2d1ME5UTmNHZU1kYTRD?=
 =?utf-8?B?WlhYMkxSNElqeHJOMFlaVzlLdEcraTN2QlE4K3RNMlhXeGRURFYvSEF3cFpv?=
 =?utf-8?B?enBtcktwbFlMSWw4a3lWV251V0p5SHdyTEJjb0xhcUxSbkFqbzZySDBqaVFG?=
 =?utf-8?B?TlZoVGkzYk1Sc1JVdjE4dHhLMlQxcTlLWGJyL0FXL2MwRGhLRmFYS1RpK2Mv?=
 =?utf-8?B?cjY3aHVIdTdjZmlFdmdDcEh6ZjBtQStkSjFVY2pwaGlUVWNta3E1NE9OdDRD?=
 =?utf-8?B?OEh6N1d0c2dDY2NQSEZLc2laNmdsY0pHanNIK2VraUgzM3hEM2syVWFtVkc5?=
 =?utf-8?B?cDdrZzVyaDlmTlVUNTlSOWZPeEEyK2xjaVNiZjljL210ZzRONGpFMU9Wdi9N?=
 =?utf-8?B?eXRsWk16WnZPV1pkVnA3c1pUUHJpZXBBSitzdWdZK0VvV0twcTRkOEo3a1Rj?=
 =?utf-8?B?d3RtWlBDUld4TUJzUUNLRis4Y0xJby85VmRYcjJ5WlZPdE00anN4TStYTnhT?=
 =?utf-8?B?eW1sUDJCQUh0OElUR29oZXpzZ2hJTGhzalJWeFJSRzd3KzBWMkZpRUJMSnZO?=
 =?utf-8?B?by8wRDRDZ1Q1c1NqTVlVTWRmWTl1RkoxNm9NcFRzcU5iVXh2VS9VeUpIMTZ2?=
 =?utf-8?B?cjV4TFRhSjMzTXZPeG5aaUc3NEZrTWlsdEx6Lyt6NDRBbGdkR0F4RGc2N1E0?=
 =?utf-8?B?OEwzNkExWDdlRkhuYkNQRGZtamZZQUVqUE1qMnBwRmJCYk5sS1piTERmS3lp?=
 =?utf-8?B?ZFpvR3pZK1JnbUVsZmsxUGthS1lNOVdJSzBKZUpGUEdRaHMzSDBSYmp3Y1Zj?=
 =?utf-8?B?MUI1ckhIN1JOQkdYaXdFaTEyVDhidExQdjBRVGdNSVBXcHRoNDZyZTdyRTJM?=
 =?utf-8?B?RTBqcFVBaExZWXdTQWJ4bDR4anZFVW9NeHNpMnlxaXdFYUxUKzlOVmpubzBB?=
 =?utf-8?B?d2FSK05ERFdlRjJ2RjFaOHQyZGM3TzFBS3gwd0I2TEd2RHBoY0pzdjZWSEtn?=
 =?utf-8?B?eGlRd1hpcEJEU3lVN0JhbDZheUNoN1A4UlByMVhKcmx6SHhxdkFCbHdFc1NB?=
 =?utf-8?B?QmM5ak53dlljT0N3VnNBZVlrWjNkdjBOYlg1U1hSUXIyekxlVnBJa2tGUWU0?=
 =?utf-8?B?S2hQYUhuY2g0MU8xRUtCNEF1dk5URUFwM3BkdWhRb2pMU2J1aUMyaFRqa3lU?=
 =?utf-8?B?U2dtMnlITjVSQlJjYUpGUWszTkFhd0I5OHdCSVBkTitCOG40TFZNTFRSMkp4?=
 =?utf-8?B?N1NNSVd1eERKVFB5L3dRdFcyck5NSUJqRW1hYWc2SFJOMFMzdnVMRTJxa0hK?=
 =?utf-8?Q?NR36zzsc+5AIzb34udo7qtZkTy+VsDr3iwLtmnWTrqjT?=
X-OriginatorOrg: amd.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 58a2fea0-7a65-490d-019b-08da8f6c3131
X-MS-Exchange-CrossTenant-AuthSource: BN8PR12MB3587.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 05 Sep 2022 18:26:47.3693
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: /+mmSSUxF8Ny4Qk/IpGZJiFqUWrbZYg8k5enzf7AXUwWU3fNpHyj3eqxDc+SooK4
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR12MB4485
X-Spam-Status: No, score=-3.8 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,
        RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_PASS,T_SCC_BODY_TEXT_LINE
        autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Am 05.09.22 um 18:39 schrieb Tvrtko Ursulin:
>
> On 05/09/2022 12:21, Christian König wrote:
>> Am 05.09.22 um 12:56 schrieb Arvind Yadav:
>>> The core DMA-buf framework needs to enable signaling
>>> before the fence is signaled. The core DMA-buf framework
>>> can forget to enable signaling before the fence is signaled.
>>> To avoid this scenario on the debug kernel, check the
>>> DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT status bit before checking
>>> the signaling bit status to confirm that enable_signaling
>>> is enabled.
>>
>> You might want to put this patch at the end of the series to avoid 
>> breaking the kernel in between.
>>
>>>
>>> Signed-off-by: Arvind Yadav <Arvind.Yadav@amd.com>
>>> ---
>>>   include/linux/dma-fence.h | 5 +++++
>>>   1 file changed, 5 insertions(+)
>>>
>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>>> index 775cdc0b4f24..60c0e935c0b5 100644
>>> --- a/include/linux/dma-fence.h
>>> +++ b/include/linux/dma-fence.h
>>> @@ -428,6 +428,11 @@ dma_fence_is_signaled_locked(struct dma_fence 
>>> *fence)
>>>   static inline bool
>>>   dma_fence_is_signaled(struct dma_fence *fence)
>>>   {
>>> +#ifdef CONFIG_DEBUG_FS
>>
>> CONFIG_DEBUG_FS is certainly wrong, probably better to check for 
>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH here.
>>
>> Apart from that looks good to me,
>
> What's the full story in this series - I'm afraid the cover letter 
> does not make it clear to a casual reader like myself? Where does the 
> difference between debug and non debug kernel come from?

We have a bug that the drm_sync file doesn't properly enable signaling 
leading to an igt test failure.

>
> And how do the proposed changes relate to the following kerneldoc 
> excerpt:
>
>      * Since many implementations can call dma_fence_signal() even 
> when before
>      * @enable_signaling has been called there's a race window, where the
>      * dma_fence_signal() might result in the final fence reference being
>      * released and its memory freed. To avoid this, implementations 
> of this
>      * callback should grab their own reference using dma_fence_get(), 
> to be
>      * released when the fence is signalled (through e.g. the interrupt
>      * handler).
>      *
>      * This callback is optional. If this callback is not present, 
> then the
>      * driver must always have signaling enabled.
>
> Is it now an error, or should be impossible condition, for "is 
> signaled" to return true _unless_ signaling has been enabled?

That's neither an error nor impossible. For debugging we just never 
return signaled from the dma_fence_is_signaled() function when signaling 
was not enabled before.

I also plan to remove the return value from the enable_signaling 
callback. That was just not very well designed.

>
> If the statement (in a later patch) is signalling should always be 
> explicitly enabled by the callers of dma_fence_add_callback, then what 
> about the existing call to __dma_fence_enable_signaling from 
> dma_fence_add_callback?

Oh, good point. That sounds like we have some bug in the core dma_fence 
code as well.

Calls to dma_fence_add_callback() and dma_fence_wait() should enable 
signaling implicitly and don't need an extra call for that.

Only dma_fence_is_signaled() needs this explicit enabling of signaling 
through dma_fence_enable_sw_signaling().

>
> Or if the rules are changing shouldn't kerneldoc be updated as part of 
> the series?

I think the kerneldoc is just a bit misleading. The point is that when 
you need to call dma_fence_enable_sw_signaling() you must hold a 
reference to the fence object.

But that's true for all the dma_fence_* functions. The race described in 
the comment is just nonsense because you need to hold that reference anyway.

Regards,
Christian.

>
> Regards,
>
> Tvrtko
>
>> Christian.
>>
>>> +    if (!test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags))
>>> +        return false;
>>> +#endif
>>> +
>>>       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>>>           return true;
>>