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

[Danilo Krummrich]

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 at 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 at 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.