drm/sched: fix potential page fault in drm_sched_job_init()

  Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
number of run-queues") introduces drm_err() in drm_sched_job_init(), in
order to indicate that the given entity has no runq, however at this
time job->sched is not yet set, likely to be NULL initialized, and hence
shouldn't be used.

Replace the corresponding drm_err() call with pr_err() to avoid a
potential page fault.

While at it, extend the documentation of drm_sched_job_init() to
indicate that job->sched is not a valid pointer until
drm_sched_job_arm() has been called.

Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
---
 drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)


base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
  

Hi,

Could you please use my gmail address, the one one I'm responding from--I don't want
to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
as undeliverable.

On 2023-11-07 21:26, Danilo Krummrich wrote:
> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
> order to indicate that the given entity has no runq, however at this
> time job->sched is not yet set, likely to be NULL initialized, and hence
> shouldn't be used.
> 
> Replace the corresponding drm_err() call with pr_err() to avoid a
> potential page fault.
> 
> While at it, extend the documentation of drm_sched_job_init() to
> indicate that job->sched is not a valid pointer until
> drm_sched_job_arm() has been called.
> 
> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 27843e37d9b7..dd28389f0ddd 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>   * This function returns -ENOENT in this case (which probably should be -EIO as
>   * a more meanigful return value).
>   *
> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
> + * been called.
> + *

Good catch!

Did you actually get this to page-fault and have a kernel log?

I'm asking because we see it correctly set in this kernel log coming from AMD,

[   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!

in this email,
https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com

>   * Returns 0 for success, negative error code otherwise.
>   */
>  int drm_sched_job_init(struct drm_sched_job *job,
> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>  		 * or worse--a blank screen--leave a trail in the
>  		 * logs, so this can be debugged easier.
>  		 */
> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
> +		pr_err("%s: entity has no rq!\n", __func__);

Is it feasible to do something like the following?

		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);

>  		return -ENOENT;
>  	}
>  
> 
> base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
  

On 2023-11-08 00:46, Luben Tuikov wrote:
> Hi,
> 
> Could you please use my gmail address, the one one I'm responding from--I don't want
> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
> as undeliverable.
> 
> On 2023-11-07 21:26, Danilo Krummrich wrote:
>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>> order to indicate that the given entity has no runq, however at this
>> time job->sched is not yet set, likely to be NULL initialized, and hence
>> shouldn't be used.
>>
>> Replace the corresponding drm_err() call with pr_err() to avoid a
>> potential page fault.
>>
>> While at it, extend the documentation of drm_sched_job_init() to
>> indicate that job->sched is not a valid pointer until
>> drm_sched_job_arm() has been called.
>>
>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>> ---
>>  drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 27843e37d9b7..dd28389f0ddd 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>   * This function returns -ENOENT in this case (which probably should be -EIO as
>>   * a more meanigful return value).
>>   *
>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>> + * been called.
>> + *
> 
> Good catch!
> 
> Did you actually get this to page-fault and have a kernel log?
> 
> I'm asking because we see it correctly set in this kernel log coming from AMD,
> 
> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
> 
> in this email,
> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
> 
>>   * Returns 0 for success, negative error code otherwise.
>>   */
>>  int drm_sched_job_init(struct drm_sched_job *job,
>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>  		 * or worse--a blank screen--leave a trail in the
>>  		 * logs, so this can be debugged easier.
>>  		 */
>> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>> +		pr_err("%s: entity has no rq!\n", __func__);
> 
> Is it feasible to do something like the following?
> 
> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);

Sorry, that was meant to be like this to make the print look just like the original,

		dev_err(job->sched ? job->sched->dev : NULL, "[drm] *ERROR* %s: entity has no rq!\n", __func__);

> 
>>  		return -ENOENT;
>>  	}
>>  
>>
>> base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
>
  

On 11/8/23 06:46, Luben Tuikov wrote:
> Hi,
> 
> Could you please use my gmail address, the one one I'm responding from--I don't want
> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
> as undeliverable.
> 
> On 2023-11-07 21:26, Danilo Krummrich wrote:
>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>> order to indicate that the given entity has no runq, however at this
>> time job->sched is not yet set, likely to be NULL initialized, and hence
>> shouldn't be used.
>>
>> Replace the corresponding drm_err() call with pr_err() to avoid a
>> potential page fault.
>>
>> While at it, extend the documentation of drm_sched_job_init() to
>> indicate that job->sched is not a valid pointer until
>> drm_sched_job_arm() has been called.
>>
>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 27843e37d9b7..dd28389f0ddd 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>    * a more meanigful return value).
>>    *
>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>> + * been called.
>> + *
> 
> Good catch!
> 
> Did you actually get this to page-fault and have a kernel log?

No, I just found it because I was about to make the same mistake.

> 
> I'm asking because we see it correctly set in this kernel log coming from AMD,

I think that's because amdgpu just sets job->sched to *some* scheduler instance after
job allocation [1].

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108

> 
> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
> 
> in this email,
> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
> 
>>    * Returns 0 for success, negative error code otherwise.
>>    */
>>   int drm_sched_job_init(struct drm_sched_job *job,
>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>   		 * or worse--a blank screen--leave a trail in the
>>   		 * logs, so this can be debugged easier.
>>   		 */
>> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>> +		pr_err("%s: entity has no rq!\n", __func__);
> 
> Is it feasible to do something like the following?
> 
> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);

I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
drm_sched_job_arm() is called.

However, I notice there are quite a view more fields of struct drm_sched_job that are never
initialized, hence there are either a couple more potential bugs or missing documentation that
drivers *must* ensure that a job is zero-initialized.

Not quite sure if we really want to rely on the latter for core infrastructure...

> 
>>   		return -ENOENT;
>>   	}
>>   
>>
>> base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
>
  

On 2023-11-08 19:09, Danilo Krummrich wrote:
> On 11/8/23 06:46, Luben Tuikov wrote:
>> Hi,
>>
>> Could you please use my gmail address, the one one I'm responding from--I don't want
>> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
>> as undeliverable.
>>
>> On 2023-11-07 21:26, Danilo Krummrich wrote:
>>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>>> order to indicate that the given entity has no runq, however at this
>>> time job->sched is not yet set, likely to be NULL initialized, and hence
>>> shouldn't be used.
>>>
>>> Replace the corresponding drm_err() call with pr_err() to avoid a
>>> potential page fault.
>>>
>>> While at it, extend the documentation of drm_sched_job_init() to
>>> indicate that job->sched is not a valid pointer until
>>> drm_sched_job_arm() has been called.
>>>
>>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>>> ---
>>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 27843e37d9b7..dd28389f0ddd 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>>    * a more meanigful return value).
>>>    *
>>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>>> + * been called.
>>> + *
>>
>> Good catch!
>>
>> Did you actually get this to page-fault and have a kernel log?
> 
> No, I just found it because I was about to make the same mistake.
> 
>>
>> I'm asking because we see it correctly set in this kernel log coming from AMD,
> 
> I think that's because amdgpu just sets job->sched to *some* scheduler instance after
> job allocation [1].
> 
> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108
> 
>>
>> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
>>
>> in this email,
>> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
>>
>>>    * Returns 0 for success, negative error code otherwise.
>>>    */
>>>   int drm_sched_job_init(struct drm_sched_job *job,
>>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>   		 * or worse--a blank screen--leave a trail in the
>>>   		 * logs, so this can be debugged easier.
>>>   		 */
>>> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>>> +		pr_err("%s: entity has no rq!\n", __func__);
>>
>> Is it feasible to do something like the following?
>>
>> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);
> 
> I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
> structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
> drm_sched_job_arm() is called.

Okay. However, when using pr_err() we're losing "[drm] *ERROR* " prefix and we scan for that
in the logs to quickly find the cause of the error.

Perhaps we can define pr_fmt() and also include "*ERROR*" so that we can get the desired result
as the attached patch shows?
  

On 11/9/23 05:23, Luben Tuikov wrote:
> On 2023-11-08 19:09, Danilo Krummrich wrote:
>> On 11/8/23 06:46, Luben Tuikov wrote:
>>> Hi,
>>>
>>> Could you please use my gmail address, the one one I'm responding from--I don't want
>>> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
>>> as undeliverable.
>>>
>>> On 2023-11-07 21:26, Danilo Krummrich wrote:
>>>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>>>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>>>> order to indicate that the given entity has no runq, however at this
>>>> time job->sched is not yet set, likely to be NULL initialized, and hence
>>>> shouldn't be used.
>>>>
>>>> Replace the corresponding drm_err() call with pr_err() to avoid a
>>>> potential page fault.
>>>>
>>>> While at it, extend the documentation of drm_sched_job_init() to
>>>> indicate that job->sched is not a valid pointer until
>>>> drm_sched_job_arm() has been called.
>>>>
>>>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>>>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>>>> ---
>>>>    drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 27843e37d9b7..dd28389f0ddd 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>>>     * This function returns -ENOENT in this case (which probably should be -EIO as
>>>>     * a more meanigful return value).
>>>>     *
>>>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>>>> + * been called.
>>>> + *
>>>
>>> Good catch!
>>>
>>> Did you actually get this to page-fault and have a kernel log?
>>
>> No, I just found it because I was about to make the same mistake.
>>
>>>
>>> I'm asking because we see it correctly set in this kernel log coming from AMD,
>>
>> I think that's because amdgpu just sets job->sched to *some* scheduler instance after
>> job allocation [1].
>>
>> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108
>>
>>>
>>> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
>>>
>>> in this email,
>>> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
>>>
>>>>     * Returns 0 for success, negative error code otherwise.
>>>>     */
>>>>    int drm_sched_job_init(struct drm_sched_job *job,
>>>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>>    		 * or worse--a blank screen--leave a trail in the
>>>>    		 * logs, so this can be debugged easier.
>>>>    		 */
>>>> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>>>> +		pr_err("%s: entity has no rq!\n", __func__);
>>>
>>> Is it feasible to do something like the following?
>>>
>>> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);
>>
>> I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
>> structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
>> drm_sched_job_arm() is called.
> 
> Okay. However, when using pr_err() we're losing "[drm] *ERROR* " prefix and we scan for that
> in the logs to quickly find the cause of the error.
> 
> Perhaps we can define pr_fmt() and also include "*ERROR*" so that we can get the desired result
> as the attached patch shows?

Sure, I'd add the pr_fmt() in a separate patch though.
  

On 11/9/23 01:09, Danilo Krummrich wrote:
> On 11/8/23 06:46, Luben Tuikov wrote:
>> Hi,
>>
>> Could you please use my gmail address, the one one I'm responding from--I don't want
>> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
>> as undeliverable.
>>
>> On 2023-11-07 21:26, Danilo Krummrich wrote:
>>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>>> order to indicate that the given entity has no runq, however at this
>>> time job->sched is not yet set, likely to be NULL initialized, and hence
>>> shouldn't be used.
>>>
>>> Replace the corresponding drm_err() call with pr_err() to avoid a
>>> potential page fault.
>>>
>>> While at it, extend the documentation of drm_sched_job_init() to
>>> indicate that job->sched is not a valid pointer until
>>> drm_sched_job_arm() has been called.
>>>
>>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>>> ---
>>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 27843e37d9b7..dd28389f0ddd 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>>    * a more meanigful return value).
>>>    *
>>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>>> + * been called.
>>> + *
>>
>> Good catch!
>>
>> Did you actually get this to page-fault and have a kernel log?
> 
> No, I just found it because I was about to make the same mistake.
> 
>>
>> I'm asking because we see it correctly set in this kernel log coming from AMD,
> 
> I think that's because amdgpu just sets job->sched to *some* scheduler instance after
> job allocation [1].
> 
> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108
> 
>>
>> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
>>
>> in this email,
>> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
>>
>>>    * Returns 0 for success, negative error code otherwise.
>>>    */
>>>   int drm_sched_job_init(struct drm_sched_job *job,
>>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>            * or worse--a blank screen--leave a trail in the
>>>            * logs, so this can be debugged easier.
>>>            */
>>> -        drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>>> +        pr_err("%s: entity has no rq!\n", __func__);
>>
>> Is it feasible to do something like the following?
>>
>>         dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);
> 
> I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
> structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
> drm_sched_job_arm() is called.
> 
> However, I notice there are quite a view more fields of struct drm_sched_job that are never
> initialized, hence there are either a couple more potential bugs or missing documentation that
> drivers *must* ensure that a job is zero-initialized.

Any opinions on that? Otherwise I'd probably go ahead and send a fix for the other bugs too.

> 
> Not quite sure if we really want to rely on the latter for core infrastructure...
> 
>>
>>>           return -ENOENT;
>>>       }
>>>
>>> base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
>>
  

On 2023-11-09 14:55, Danilo Krummrich wrote:
> On 11/9/23 01:09, Danilo Krummrich wrote:
>> On 11/8/23 06:46, Luben Tuikov wrote:
>>> Hi,
>>>
>>> Could you please use my gmail address, the one one I'm responding from--I don't want
>>> to miss any DRM scheduler patches. BTW, the luben.tuikov@amd.com email should bounce
>>> as undeliverable.
>>>
>>> On 2023-11-07 21:26, Danilo Krummrich wrote:
>>>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>>>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>>>> order to indicate that the given entity has no runq, however at this
>>>> time job->sched is not yet set, likely to be NULL initialized, and hence
>>>> shouldn't be used.
>>>>
>>>> Replace the corresponding drm_err() call with pr_err() to avoid a
>>>> potential page fault.
>>>>
>>>> While at it, extend the documentation of drm_sched_job_init() to
>>>> indicate that job->sched is not a valid pointer until
>>>> drm_sched_job_arm() has been called.
>>>>
>>>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>>>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>>>> ---
>>>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 27843e37d9b7..dd28389f0ddd 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>>>    * a more meanigful return value).
>>>>    *
>>>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>>>> + * been called.
>>>> + *
>>>
>>> Good catch!
>>>
>>> Did you actually get this to page-fault and have a kernel log?
>>
>> No, I just found it because I was about to make the same mistake.
>>
>>>
>>> I'm asking because we see it correctly set in this kernel log coming from AMD,
>>
>> I think that's because amdgpu just sets job->sched to *some* scheduler instance after
>> job allocation [1].
>>
>> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108
>>
>>>
>>> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
>>>
>>> in this email,
>>> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
>>>
>>>>    * Returns 0 for success, negative error code otherwise.
>>>>    */
>>>>   int drm_sched_job_init(struct drm_sched_job *job,
>>>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>>            * or worse--a blank screen--leave a trail in the
>>>>            * logs, so this can be debugged easier.
>>>>            */
>>>> -        drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>>>> +        pr_err("%s: entity has no rq!\n", __func__);
>>>
>>> Is it feasible to do something like the following?
>>>
>>>         dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);
>>
>> I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
>> structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
>> drm_sched_job_arm() is called.
>>
>> However, I notice there are quite a view more fields of struct drm_sched_job that are never
>> initialized, hence there are either a couple more potential bugs or missing documentation that
>> drivers *must* ensure that a job is zero-initialized.
> 
> Any opinions on that? Otherwise I'd probably go ahead and send a fix for the other bugs too.

Send the patches.

Will those patches also add pr_fmt() for DRM?

I'm asking because you said you'll add pr_fmt() in a "separate" patch, and I thought it was
okay being self-contained in your patch as per the version I sent.