Re: [Intel-gfx] [PATCH 01/11] drm/i915: Release i915_gem_context from a worker

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 13/08/2021 21:30, Daniel Vetter wrote:
> 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.

There is reset which definitely isn't process context.

Otherwise I did not really get from the commit message is this patch 
fixing an existing problem or preparing something for the future. If the 
former then as I wrote above - I am pretty sure there are call sites 
from the tasklet already.

Regards,

Tvrtko

> 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.
> 
> 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 ++++++++++++
>   2 files changed, 23 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.
>   	 */
>