Re: [Intel-gfx] [PATCH] drm/i915: Release i915_gem_context from a worker

Maarten Lankhorst
2021-08-31T09:38:15Z

Op 14-08-2021 om 12:43 schreef Daniel Vetter:
> The only reason for this really is the i915_gem_engines->fence
> callback engines_notify(), which exists purely as a fairly funky
> reference counting scheme for that. Otherwise all other callers are
> from process context, and generally fairly benign locking context.
>
> Unfortunately untangling that requires some major surgery, and we have
> a few i915_gem_context reference counting bugs that need fixing, and
> they blow in the current hardirq calling context, so we need a
> stop-gap measure.
>
> Put a FIXME comment in when this should be removable again.
>
> v2: Fix mock_context(), noticed by intel-gfx-ci.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> ---
> drivers/gpu/drm/i915/gem/i915_gem_context.c | 13 +++++++++++--
> drivers/gpu/drm/i915/gem/i915_gem_context_types.h | 12 ++++++++++++
> drivers/gpu/drm/i915/gem/selftests/mock_context.c | 1 +
> 3 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index fd169cf2f75a..051bc357ff65 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -986,9 +986,10 @@ static struct i915_gem_engines *user_engines(struct i915_gem_context *ctx,
> return err;
> }
>
> -void i915_gem_context_release(struct kref *ref)
> +static void i915_gem_context_release_work(struct work_struct *work)
> {
> - struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
> + struct i915_gem_context *ctx = container_of(work, typeof(*ctx),
> + release_work);
>
> trace_i915_context_free(ctx);
> GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
> @@ -1002,6 +1003,13 @@ void i915_gem_context_release(struct kref *ref)
> kfree_rcu(ctx, rcu);
> }
>
> +void i915_gem_context_release(struct kref *ref)
> +{
> + struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
> +
> + queue_work(ctx->i915->wq, &ctx->release_work);
> +}
> +
> static inline struct i915_gem_engines *
> __context_engines_static(const struct i915_gem_context *ctx)
> {
> @@ -1303,6 +1311,7 @@ i915_gem_create_context(struct drm_i915_private *i915,
> ctx->sched = pc->sched;
> mutex_init(&ctx->mutex);
> INIT_LIST_HEAD(&ctx->link);
> + INIT_WORK(&ctx->release_work, i915_gem_context_release_work);
>
> spin_lock_init(&ctx->stale.lock);
> INIT_LIST_HEAD(&ctx->stale.engines);
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> index 94c03a97cb77..0c38789bd4a8 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> @@ -288,6 +288,18 @@ struct i915_gem_context {
> */
> struct kref ref;
>
> + /**
> + * @release_work:
> + *
> + * Work item for deferred cleanup, since i915_gem_context_put() tends to
> + * be called from hardirq context.
> + *
> + * FIXME: The only real reason for this is &i915_gem_engines.fence, all
> + * other callers are from process context and need at most some mild
> + * shuffling to pull the i915_gem_context_put() call out of a spinlock.
> + */
> + struct work_struct release_work;
> +
> /**
> * @rcu: rcu_head for deferred freeing.
> */
> diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_context.c b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> index fee070df1c97..067d68a6fe4c 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> @@ -23,6 +23,7 @@ mock_context(struct drm_i915_private *i915,
> kref_init(&ctx->ref);
> INIT_LIST_HEAD(&ctx->link);
> ctx->i915 = i915;
> + INIT_WORK(&ctx->release_work, i915_gem_context_release_work);
>
> mutex_init(&ctx->mutex);
>

----
Is the workqueue really needed? I'm not sure you could still race in drm_syncobj_free when refcount is zero, so in that case removing locking from _release would work as well as a workqueue.

Something like below would keep the drm_sync_obj_put hardirq safe.

I assume when freeing, the cb list is supposed to be empty, so I added a WARN_ON just to be sure, otherwise we should just tear down the list without locking too.

This should be a better alternative for patch 1.
----8<-------
diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index c9a9d74f338c..9d561decd97e 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -462,7 +462,13 @@ void drm_syncobj_free(struct kref *kref)
struct drm_syncobj *syncobj = container_of(kref,
struct drm_syncobj,
refcount);
- drm_syncobj_replace_fence(syncobj, NULL);
+ struct dma_fence *old_fence;
+
+ old_fence = rcu_dereference_protected(syncobj->fence, !kref_read(&syncobj->refcount));
+ dma_fence_put(old_fence);
+
+ WARN_ON(!list_empty(&syncobj->cb_list));
+
kfree(syncobj);
}
EXPORT_SYMBOL(drm_syncobj_free);