[PATCH v3 1/5] drm/sched: Fix teardown leaks with waitqueue

[Philipp Stanner <phasta@kernel.org>]

From: Philipp Stanner <pstanner@redhat.com>

The GPU scheduler currently does not ensure that its pending_list is
empty before performing various other teardown tasks in
drm_sched_fini().

If there are still jobs in the pending_list, this is problematic because
after scheduler teardown, no one will call backend_ops.free_job()
anymore. This would, consequently, result in memory leaks.

One way to solve this is to implement a waitqueue that drm_sched_fini()
blocks on until the pending_list has become empty. That waitqueue must
obviously not block for a significant time. Thus, it's necessary to only
wait if it's guaranteed that all fences will get signaled quickly.

This can be ensured by having the driver implement a new backend ops,
cancel_pending_fences(), in which the driver shall signal all
unsignaled, in-flight fences with an error.

Add a waitqueue to struct drm_gpu_scheduler. Wake up waiters once the
pending_list becomes empty. Wait in drm_sched_fini() for that to happen
if cancel_pending_fences() is implemented.

Signed-off-by: Philipp Stanner <pstanner@redhat.com>
---
 drivers/gpu/drm/scheduler/sched_main.c | 105 ++++++++++++++++++++-----
 include/drm/gpu_scheduler.h            |  19 +++++
 2 files changed, 105 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index f7118497e47a..406572f5168e 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -367,7 +367,7 @@ static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
  */
 static void __drm_sched_run_free_queue(struct drm_gpu_scheduler *sched)
 {
-	if (!READ_ONCE(sched->pause_submit))
+	if (!READ_ONCE(sched->pause_free))
 		queue_work(sched->submit_wq, &sched->work_free_job);
 }
 
@@ -1121,6 +1121,12 @@ drm_sched_get_finished_job(struct drm_gpu_scheduler *sched)
 		/* remove job from pending_list */
 		list_del_init(&job->list);
 
+		/*
+		 * Inform tasks blocking in drm_sched_fini() that it's now safe to proceed.
+		 */
+		if (list_empty(&sched->pending_list))
+			wake_up(&sched->pending_list_waitque);
+
 		/* cancel this job's TO timer */
 		cancel_delayed_work(&sched->work_tdr);
 		/* make the scheduled timestamp more accurate */
@@ -1326,6 +1332,7 @@ int drm_sched_init(struct drm_gpu_scheduler *sched, const struct drm_sched_init_
 	init_waitqueue_head(&sched->job_scheduled);
 	INIT_LIST_HEAD(&sched->pending_list);
 	spin_lock_init(&sched->job_list_lock);
+	init_waitqueue_head(&sched->pending_list_waitque);
 	atomic_set(&sched->credit_count, 0);
 	INIT_DELAYED_WORK(&sched->work_tdr, drm_sched_job_timedout);
 	INIT_WORK(&sched->work_run_job, drm_sched_run_job_work);
@@ -1333,6 +1340,7 @@ int drm_sched_init(struct drm_gpu_scheduler *sched, const struct drm_sched_init_
 	atomic_set(&sched->_score, 0);
 	atomic64_set(&sched->job_id_count, 0);
 	sched->pause_submit = false;
+	sched->pause_free = false;
 
 	sched->ready = true;
 	return 0;
@@ -1350,33 +1358,90 @@ int drm_sched_init(struct drm_gpu_scheduler *sched, const struct drm_sched_init_
 }
 EXPORT_SYMBOL(drm_sched_init);
 
+/**
+ * drm_sched_submission_and_timeout_stop - stop everything except for free_job
+ * @sched: scheduler instance
+ *
+ * Helper for tearing down the scheduler in drm_sched_fini().
+ */
+static void
+drm_sched_submission_and_timeout_stop(struct drm_gpu_scheduler *sched)
+{
+	WRITE_ONCE(sched->pause_submit, true);
+	cancel_work_sync(&sched->work_run_job);
+	cancel_delayed_work_sync(&sched->work_tdr);
+}
+
+/**
+ * drm_sched_free_stop - stop free_job
+ * @sched: scheduler instance
+ *
+ * Helper for tearing down the scheduler in drm_sched_fini().
+ */
+static void drm_sched_free_stop(struct drm_gpu_scheduler *sched)
+{
+	WRITE_ONCE(sched->pause_free, true);
+	cancel_work_sync(&sched->work_free_job);
+}
+
+/**
+ * drm_sched_no_jobs_pending - check whether jobs are pending
+ * @sched: scheduler instance
+ *
+ * Checks if jobs are pending for @sched.
+ *
+ * Return: true if jobs are pending, false otherwise.
+ */
+static bool drm_sched_no_jobs_pending(struct drm_gpu_scheduler *sched)
+{
+	bool empty;
+
+	spin_lock(&sched->job_list_lock);
+	empty = list_empty(&sched->pending_list);
+	spin_unlock(&sched->job_list_lock);
+
+	return empty;
+}
+
+/**
+ * drm_sched_cancel_jobs_and_wait - trigger freeing of all pending jobs
+ * @sched: scheduler instance
+ *
+ * Must only be called if &struct drm_sched_backend_ops.cancel_pending_fences is
+ * implemented.
+ *
+ * Instructs the driver to kill the fence context associated with this scheduler,
+ * thereby signaling all pending fences. This, in turn, will trigger
+ * &struct drm_sched_backend_ops.free_job to be called for all pending jobs.
+ * The function then blocks until all pending jobs have been freed.
+ */
+static void drm_sched_cancel_jobs_and_wait(struct drm_gpu_scheduler *sched)
+{
+	sched->ops->cancel_pending_fences(sched);
+	wait_event(sched->pending_list_waitque, drm_sched_no_jobs_pending(sched));
+}
+
 /**
  * drm_sched_fini - Destroy a gpu scheduler
  *
  * @sched: scheduler instance
  *
- * Tears down and cleans up the scheduler.
- *
- * This stops submission of new jobs to the hardware through
- * drm_sched_backend_ops.run_job(). Consequently, drm_sched_backend_ops.free_job()
- * will not be called for all jobs still in drm_gpu_scheduler.pending_list.
- * There is no solution for this currently. Thus, it is up to the driver to make
- * sure that:
- *
- *  a) drm_sched_fini() is only called after for all submitted jobs
- *     drm_sched_backend_ops.free_job() has been called or that
- *  b) the jobs for which drm_sched_backend_ops.free_job() has not been called
- *     after drm_sched_fini() ran are freed manually.
- *
- * FIXME: Take care of the above problem and prevent this function from leaking
- * the jobs in drm_gpu_scheduler.pending_list under any circumstances.
+ * Tears down and cleans up the scheduler. Might leak memory if
+ * &struct drm_sched_backend_ops.cancel_pending_fences is not implemented.
  */
 void drm_sched_fini(struct drm_gpu_scheduler *sched)
 {
 	struct drm_sched_entity *s_entity;
 	int i;
 
-	drm_sched_wqueue_stop(sched);
+	if (sched->ops->cancel_pending_fences) {
+		drm_sched_submission_and_timeout_stop(sched);
+		drm_sched_cancel_jobs_and_wait(sched);
+		drm_sched_free_stop(sched);
+	} else {
+		/* We're in "legacy free-mode" and ignore potential mem leaks */
+		drm_sched_wqueue_stop(sched);
+	}
 
 	for (i = DRM_SCHED_PRIORITY_KERNEL; i < sched->num_rqs; i++) {
 		struct drm_sched_rq *rq = sched->sched_rq[i];
@@ -1464,7 +1529,7 @@ bool drm_sched_wqueue_ready(struct drm_gpu_scheduler *sched)
 EXPORT_SYMBOL(drm_sched_wqueue_ready);
 
 /**
- * drm_sched_wqueue_stop - stop scheduler submission
+ * drm_sched_wqueue_stop - stop scheduler submission and freeing
  * @sched: scheduler instance
  *
  * Stops the scheduler from pulling new jobs from entities. It also stops
@@ -1473,13 +1538,14 @@ EXPORT_SYMBOL(drm_sched_wqueue_ready);
 void drm_sched_wqueue_stop(struct drm_gpu_scheduler *sched)
 {
 	WRITE_ONCE(sched->pause_submit, true);
+	WRITE_ONCE(sched->pause_free, true);
 	cancel_work_sync(&sched->work_run_job);
 	cancel_work_sync(&sched->work_free_job);
 }
 EXPORT_SYMBOL(drm_sched_wqueue_stop);
 
 /**
- * drm_sched_wqueue_start - start scheduler submission
+ * drm_sched_wqueue_start - start scheduler submission and freeing
  * @sched: scheduler instance
  *
  * Restarts the scheduler after drm_sched_wqueue_stop() has stopped it.
@@ -1490,6 +1556,7 @@ EXPORT_SYMBOL(drm_sched_wqueue_stop);
 void drm_sched_wqueue_start(struct drm_gpu_scheduler *sched)
 {
 	WRITE_ONCE(sched->pause_submit, false);
+	WRITE_ONCE(sched->pause_free, false);
 	queue_work(sched->submit_wq, &sched->work_run_job);
 	queue_work(sched->submit_wq, &sched->work_free_job);
 }
diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index d860db087ea5..d8bd5b605336 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -29,6 +29,7 @@
 #include <linux/completion.h>
 #include <linux/xarray.h>
 #include <linux/workqueue.h>
+#include <linux/wait.h>
 
 #define MAX_WAIT_SCHED_ENTITY_Q_EMPTY msecs_to_jiffies(1000)
 
@@ -508,6 +509,19 @@ struct drm_sched_backend_ops {
          * and it's time to clean it up.
 	 */
 	void (*free_job)(struct drm_sched_job *sched_job);
+
+	/**
+	 * @cancel_pending_fences: cancel all unsignaled hardware fences
+	 *
+	 * This callback must signal all unsignaled hardware fences associated
+	 * with @sched with an appropriate error code (e.g., -ECANCELED). This
+	 * ensures that all jobs will get freed by the scheduler before
+	 * teardown.
+	 *
+	 * This callback is optional, but it is highly recommended to implement
+	 * it to avoid memory leaks.
+	 */
+	void (*cancel_pending_fences)(struct drm_gpu_scheduler *sched);
 };
 
 /**
@@ -533,6 +547,8 @@ struct drm_sched_backend_ops {
  *            timeout interval is over.
  * @pending_list: the list of jobs which are currently in the job queue.
  * @job_list_lock: lock to protect the pending_list.
+ * @pending_list_waitque: a waitqueue for drm_sched_fini() to block on until all
+ *		          pending jobs have been finished.
  * @hang_limit: once the hangs by a job crosses this limit then it is marked
  *              guilty and it will no longer be considered for scheduling.
  * @score: score to help loadbalancer pick a idle sched
@@ -540,6 +556,7 @@ struct drm_sched_backend_ops {
  * @ready: marks if the underlying HW is ready to work
  * @free_guilty: A hit to time out handler to free the guilty job.
  * @pause_submit: pause queuing of @work_run_job on @submit_wq
+ * @pause_free: pause queueing of @work_free_job on @submit_wq
  * @own_submit_wq: scheduler owns allocation of @submit_wq
  * @dev: system &struct device
  *
@@ -562,12 +579,14 @@ struct drm_gpu_scheduler {
 	struct delayed_work		work_tdr;
 	struct list_head		pending_list;
 	spinlock_t			job_list_lock;
+	wait_queue_head_t		pending_list_waitque;
 	int				hang_limit;
 	atomic_t                        *score;
 	atomic_t                        _score;
 	bool				ready;
 	bool				free_guilty;
 	bool				pause_submit;
+	bool				pause_free;
 	bool				own_submit_wq;
 	struct device			*dev;
 };
-- 
2.49.0