Re: [PATCH 02/11] drm/panfrost: Remove sched_lock

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, May 21, 2021 at 11:32:48AM +0200, Lucas Stach wrote:
> Am Freitag, dem 21.05.2021 um 11:09 +0200 schrieb Daniel Vetter:
> > Scheduler takes care of its own locking, dont worry. For everything else
> > there's reservation locking on each bo.
> > 
> > So seems to be entirely redundnant and just a bit in the way.
> 
> I haven't read all the surrounding code, but this looks wrong from a
> glance. You must hold a lock across drm_sched_job_init ->
> drm_sched_entity_push_job as the scheduler fences are initialized in
> the job init, so if there's no exclusive section across those two
> function calls you might end up with jobs being queued with their fence
> seqnos not monotonically increasing, which breaks all kinds of other
> stuff.

Uh indeed. That's a bit a loaded gun since generally _init() shouldn't
have any such side effects.

> I don't see a reason to hold the lock across the reservation calls,
> though.

Ok I'll adjust the patch.
-Daniel

> 
> Regards,
> Lucas
> 
> > 
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Rob Herring <robh@kernel.org>
> > Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> > Cc: Steven Price <steven.price@arm.com>
> > Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> > ---
> >  drivers/gpu/drm/panfrost/panfrost_device.c |  1 -
> >  drivers/gpu/drm/panfrost/panfrost_device.h |  2 --
> >  drivers/gpu/drm/panfrost/panfrost_job.c    | 13 ++-----------
> >  3 files changed, 2 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_device.c b/drivers/gpu/drm/panfrost/panfrost_device.c
> > index 125ed973feaa..23070c01c63f 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_device.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_device.c
> > @@ -199,7 +199,6 @@ int panfrost_device_init(struct panfrost_device *pfdev)
> >  	int err;
> >  	struct resource *res;
> >  
> > -	mutex_init(&pfdev->sched_lock);
> >  	INIT_LIST_HEAD(&pfdev->scheduled_jobs);
> >  	INIT_LIST_HEAD(&pfdev->as_lru_list);
> >  
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
> > index 597cf1459b0a..7519903bb031 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_device.h
> > +++ b/drivers/gpu/drm/panfrost/panfrost_device.h
> > @@ -105,8 +105,6 @@ struct panfrost_device {
> >  
> >  	struct panfrost_perfcnt *perfcnt;
> >  
> > -	struct mutex sched_lock;
> > -
> >  	struct {
> >  		struct work_struct work;
> >  		atomic_t pending;
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> > index 6003cfeb1322..f5d39ee14ab5 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> > @@ -218,26 +218,19 @@ static void panfrost_attach_object_fences(struct drm_gem_object **bos,
> >  
> >  int panfrost_job_push(struct panfrost_job *job)
> >  {
> > -	struct panfrost_device *pfdev = job->pfdev;
> >  	int slot = panfrost_job_get_slot(job);
> >  	struct drm_sched_entity *entity = &job->file_priv->sched_entity[slot];
> >  	struct ww_acquire_ctx acquire_ctx;
> >  	int ret = 0;
> >  
> > -	mutex_lock(&pfdev->sched_lock);
> > -
> >  	ret = drm_gem_lock_reservations(job->bos, job->bo_count,
> >  					    &acquire_ctx);
> > -	if (ret) {
> > -		mutex_unlock(&pfdev->sched_lock);
> > +	if (ret)
> >  		return ret;
> > -	}
> >  
> >  	ret = drm_sched_job_init(&job->base, entity, NULL);
> > -	if (ret) {
> > -		mutex_unlock(&pfdev->sched_lock);
> > +	if (ret)
> >  		goto unlock;
> > -	}
> >  
> >  	job->render_done_fence = dma_fence_get(&job->base.s_fence->finished);
> >  
> > @@ -248,8 +241,6 @@ int panfrost_job_push(struct panfrost_job *job)
> >  
> >  	drm_sched_entity_push_job(&job->base, entity);
> >  
> > -	mutex_unlock(&pfdev->sched_lock);
> > -
> >  	panfrost_attach_object_fences(job->bos, job->bo_count,
> >  				      job->render_done_fence);
> >  
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch