Re: [PATCH 4/5] drm/sched: Partial revert of 'drm/sched: Keep s_fence->parent pointer'

[Christian König]

Am 21.06.22 um 00:03 schrieb Andrey Grodzovsky:

Problem:
This patch caused negative refcount as described in [1] because
for that case parent fence did not signal by the time of drm_sched_stop and 
hence
kept in pending list the assumption was they will not signal and
so fence was put to account for the s_fence->parent refcount but for
amdgpu which has embedded HW fence (always same parent fence)
drm_sched_fence_release_scheduled was always called and would
still drop the count for parent fence once more. For jobs that
never signaled this imbalance was masked by refcount bug in
amdgpu_fence_driver_clear_job_fences that would not drop
refcount on the fences that were removed from fence drive
fences array (against prevois insertion into the array in
get in amdgpu_fence_emit).

Fix:
Revert this patch and by setting s_job->s_fence->parent to NULL
as before prevent the extra refcount drop in amdgpu when
drm_sched_fence_release_scheduled is called on job release.

Also - align behaviour in drm_sched_resubmit_jobs_ext with that of
drm_sched_main when submitting jobs - take a refcount for the
new parent fence pointer and drop refcount for original kref_init
for new HW fence creation (or fake new HW fence in amdgpu - see next patch).

[1] - 
https://lore.kernel.org/all/731b7ff1-3cc9-e314-df2a-7c51b76d4...@amd.com/t/#r00c728fcc069b1276642c325bfa9d82bf8fa21a3

Signed-off-by: Andrey Grodzovsky <andrey.grodzov...@amd.com>
Tested-by: Yiqing Yao <yiqing....@amd.com>
---
  drivers/gpu/drm/scheduler/sched_main.c | 16 +++++++++++++---
  1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
b/drivers/gpu/drm/scheduler/sched_main.c
index b81fceb0b8a2..b38394f5694f 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -419,6 +419,11 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, 
struct drm_sched_job *bad)
                if (s_job->s_fence->parent &&
                    dma_fence_remove_callback(s_job->s_fence->parent,
                                              &s_job->cb)) {
+                       /* Revert drm/sched: Keep s_fence->parent pointer, no
+                        * need anymore for amdgpu and creates only troubles
+                        */

Please no amdgpu specific comments in common code.

Apart from that looks like a step into the right direction to me.

Regards,
Christian.

+                       dma_fence_put(s_job->s_fence->parent);
+                       s_job->s_fence->parent = NULL;
                        atomic_dec(&sched->hw_rq_count);
                } else {
                        /*
@@ -548,7 +553,6 @@ void drm_sched_resubmit_jobs_ext(struct drm_gpu_scheduler 
*sched, int max)
                if (found_guilty && s_job->s_fence->scheduled.context == 
guilty_context)
                        dma_fence_set_error(&s_fence->finished, -ECANCELED);

- dma_fence_put(s_job->s_fence->parent);

                fence = sched->ops->run_job(s_job);
                i++;

@@ -558,7 +562,11 @@ void drm_sched_resubmit_jobs_ext(struct drm_gpu_scheduler *sched, int max) s_job->s_fence->parent = NULL;

                } else {
-                       s_job->s_fence->parent = fence;
+
+                       s_job->s_fence->parent = dma_fence_get(fence);
+
+                       /* Drop for orignal kref_init */
+                       dma_fence_put(fence);
                }
        }
  }
@@ -952,6 +960,9 @@ static int drm_sched_main(void *param)

if (!IS_ERR_OR_NULL(fence)) {

                        s_fence->parent = dma_fence_get(fence);
+                       /* Drop for original kref_init of the fence */
+                       dma_fence_put(fence);
+
                        r = dma_fence_add_callback(fence, &sched_job->cb,
                                                   drm_sched_job_done_cb);
                        if (r == -ENOENT)
@@ -959,7 +970,6 @@ static int drm_sched_main(void *param)
                        else if (r)
                                DRM_DEV_ERROR(sched->dev, "fence add callback failed 
(%d)\n",
                                          r);
-                       dma_fence_put(fence);
                } else {
                        if (IS_ERR(fence))
                                dma_fence_set_error(&s_fence->finished, 
PTR_ERR(fence));