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Message-ID: <5fb25705d202d40707730aacb6833d1939483cf4.camel@redhat.com>
Date: Mon, 30 Dec 2024 11:25:38 +0100
From: Philipp Stanner <pstanner@...hat.com>
To: Christian König <christian.koenig@....com>, Philipp
Stanner <phasta@...nel.org>, Luben Tuikov <ltuikov89@...il.com>, Matthew
Brost <matthew.brost@...el.com>, Danilo Krummrich <dakr@...nel.org>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
<mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
<airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Sumit Semwal
<sumit.semwal@...aro.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org, Tvrtko Ursulin
<tursulin@...ulin.net>, Andrey Grodzovsky <andrey.grodzovsky@....com>
Subject: Re: [PATCH] drm/sched: Document run_job() refcount hazard
On Fri, 2024-12-20 at 13:53 +0100, Christian König wrote:
> Am 20.12.24 um 13:45 schrieb Philipp Stanner:
> > From: Philipp Stanner <pstanner@...hat.com>
> >
> > drm_sched_backend_ops.run_job() returns a dma_fence for the
> > scheduler.
> > That fence is signalled by the driver once the hardware completed
> > the
> > associated job. The scheduler does not increment the reference
> > count on
> > that fence, but implicitly expects to inherit this fence from
> > run_job().
> >
> > This is relatively subtle and prone to misunderstandings.
> >
> > This implies that, to keep a reference for itself, a driver needs
> > to
> > call dma_fence_get() in addition to dma_fence_init() in that
> > callback.
> >
> > It's further complicated by the fact that the scheduler even
> > decrements
> > the refcount in drm_sched_run_job_work() since it created a new
> > reference in drm_sched_fence_scheduled(). It does, however, still
> > use
> > its pointer to the fence after calling dma_fence_put() - which is
> > safe
> > because of the aforementioned new reference, but actually still
> > violates
> > the refcounting rules.
> >
> > Improve the explanatory comment for that decrement.
> >
> > Move the call to dma_fence_put() to the position behind the last
> > usage
> > of the fence.
> >
> > Document the necessity to increment the reference count in
> > drm_sched_backend_ops.run_job().
> >
> > Cc: Christian König <christian.koenig@....com>
> > Cc: Tvrtko Ursulin <tursulin@...ulin.net>
> > Cc: Andrey Grodzovsky <andrey.grodzovsky@....com>
> > Signed-off-by: Philipp Stanner <pstanner@...hat.com>
> > ---
> > drivers/gpu/drm/scheduler/sched_main.c | 10 +++++++---
> > include/drm/gpu_scheduler.h | 20 ++++++++++++++++----
> > 2 files changed, 23 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > b/drivers/gpu/drm/scheduler/sched_main.c
> > index 7ce25281c74c..d6f8df39d848 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -1218,15 +1218,19 @@ static void drm_sched_run_job_work(struct
> > work_struct *w)
> > drm_sched_fence_scheduled(s_fence, fence);
> >
> > if (!IS_ERR_OR_NULL(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)
> > drm_sched_job_done(sched_job, fence-
> > >error);
> > else if (r)
> > DRM_DEV_ERROR(sched->dev, "fence add
> > callback failed (%d)\n", r);
> > +
> > + /*
> > + * s_fence took a new reference to fence in the
> > call to
> > + * drm_sched_fence_scheduled() above. The
> > reference passed by
> > + * run_job() above is now not needed any longer.
> > Drop it.
> > + */
> > + dma_fence_put(fence);
> > } else {
> > drm_sched_job_done(sched_job, IS_ERR(fence) ?
> > PTR_ERR(fence) : 0);
> > diff --git a/include/drm/gpu_scheduler.h
> > b/include/drm/gpu_scheduler.h
> > index 95e17504e46a..a1f5c9a14278 100644
> > --- a/include/drm/gpu_scheduler.h
> > +++ b/include/drm/gpu_scheduler.h
> > @@ -420,10 +420,22 @@ struct drm_sched_backend_ops {
> > struct drm_sched_entity
> > *s_entity);
> >
> > /**
> > - * @run_job: Called to execute the job once all of the
> > dependencies
> > - * have been resolved. This may be called multiple times,
> > if
> > - * timedout_job() has happened and
> > drm_sched_job_recovery()
> > - * decides to try it again.
> > + * @run_job: Called to execute the job once all of the
> > dependencies
> > + * have been resolved. This may be called multiple times,
> > if
> > + * timedout_job() has happened and
> > drm_sched_job_recovery() decides to
> > + * try it again.
>
> Maybe we should improve that here as well while at it.
>
> That drm_sched_job_recovery() can call this multiple times actually
> goes
> against the dma_fence rules since drivers can't easily allocate a new
> HW
> fence.
>
> Something like "The deprecated drm_sched_job_recovery() function
> might
> call this again, but it is strongly advised to not be used because it
> violates dma_fence memory allocations rules."
I just realized that drm_sched_job_recovery() is indeed deprecated so
hard that it simply doesn't exist anymore. There is no such function.
It seems to me that we (and that old docstring) are actually talking
about drm_sched_resubmit_jobs(), which is also deprecated, and which
does invoke backend_ops.run_job()?
P.
>
> On the other hand can of course be a separate patch.
>
> > + *
> > + * @sched_job: the job to run
> > + *
> > + * Returns: dma_fence the driver must signal once the
> > hardware has
> > + * completed the job ("hardware fence").
> > + *
> > + * Note that the scheduler expects to 'inherit' its own
> > reference to
> > + * this fence from the callback. It does not invoke an
> > extra
> > + * dma_fence_get() on it. Consequently, this callback must
> > return a
> > + * fence whose refcount is at least 2: One for the
> > scheduler's
> > + * reference returned here, another one for the reference
> > kept by the
> > + * driver.
>
> Well the driver actually doesn't need any extra reference. The
> scheduler
> just needs to guarantee that this reference isn't dropped before it
> is
> signaled.
>
> Regards,
> Christian.
>
> > */
> > struct dma_fence *(*run_job)(struct drm_sched_job
> > *sched_job);
> >
>
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