Re: [PATCH v4] drm/scheduler: Avoid accessing freed bad job.

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

Ping

Andrey

On 11/25/19 3:51 PM, Andrey Grodzovsky wrote:
> Problem:
> Due to a race between drm_sched_cleanup_jobs in sched thread and
> drm_sched_job_timedout in timeout work there is a possiblity that
> bad job was already freed while still being accessed from the
> timeout thread.
>
> Fix:
> Instead of just peeking at the bad job in the mirror list
> remove it from the list under lock and then put it back later when
> we are garanteed no race with main sched thread is possible which
> is after the thread is parked.
>
> v2: Lock around processing ring_mirror_list in drm_sched_cleanup_jobs.
>
> v3: Rebase on top of drm-misc-next. v2 is not needed anymore as
> drm_sched_get_cleanup_job already has a lock there.
>
> v4: Fix comments to relfect latest code in drm-misc.
>
> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
> Reviewed-by: Christian König <christian.koenig@amd.com>
> Tested-by: Emily Deng <Emily.Deng@amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 27 +++++++++++++++++++++++++++
>   1 file changed, 27 insertions(+)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 6774955..1bf9c40 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -284,10 +284,21 @@ static void drm_sched_job_timedout(struct work_struct *work)
>   	unsigned long flags;
>   
>   	sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
> +
> +	/* Protects against concurrent deletion in drm_sched_get_cleanup_job */
> +	spin_lock_irqsave(&sched->job_list_lock, flags);
>   	job = list_first_entry_or_null(&sched->ring_mirror_list,
>   				       struct drm_sched_job, node);
>   
>   	if (job) {
> +		/*
> +		 * Remove the bad job so it cannot be freed by concurrent
> +		 * drm_sched_cleanup_jobs. It will be reinserted back after sched->thread
> +		 * is parked at which point it's safe.
> +		 */
> +		list_del_init(&job->node);
> +		spin_unlock_irqrestore(&sched->job_list_lock, flags);
> +
>   		job->sched->ops->timedout_job(job);
>   
>   		/*
> @@ -298,6 +309,8 @@ static void drm_sched_job_timedout(struct work_struct *work)
>   			job->sched->ops->free_job(job);
>   			sched->free_guilty = false;
>   		}
> +	} else {
> +		spin_unlock_irqrestore(&sched->job_list_lock, flags);
>   	}
>   
>   	spin_lock_irqsave(&sched->job_list_lock, flags);
> @@ -370,6 +383,20 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
>   	kthread_park(sched->thread);
>   
>   	/*
> +	 * Reinsert back the bad job here - now it's safe as
> +	 * drm_sched_get_cleanup_job cannot race against us and release the
> +	 * bad job at this point - we parked (waited for) any in progress
> +	 * (earlier) cleanups and drm_sched_get_cleanup_job will not be called
> +	 * now until the scheduler thread is unparked.
> +	 */
> +	if (bad && bad->sched == sched)
> +		/*
> +		 * Add at the head of the queue to reflect it was the earliest
> +		 * job extracted.
> +		 */
> +		list_add(&bad->node, &sched->ring_mirror_list);
> +
> +	/*
>   	 * Iterate the job list from later to  earlier one and either deactive
>   	 * their HW callbacks or remove them from mirror list if they already
>   	 * signaled.
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