Re: [Intel-gfx] [PATCH 2/7] drm/i915/gt: Protect context lifetime with RCU

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

On 07/08/2020 09:32, Chris Wilson wrote:
> Allow a brief period for continued access to a dead intel_context by
> deferring the release of the struct until after an RCU grace period.
> As we are using a dedicated slab cache for the contexts, we can defer
> the release of the slab pages via RCU, with the caveat that individual
> structs may be reused from the freelist within an RCU grace period. To
> handle that, we have to avoid clearing members of the zombie struct.
> 
> This is required for a later patch to handle locking around virtual
> requests in the signaler, as those requests may want to move between
> engines and be destroyed while we are holding b->irq_lock on a physical
> engine.
> 
> v2: Drop mutex_reinit(), if we never mark the mutex as destroyed we
> don't need to reset the debug code, at the loss of having the mutex
> debug code spot us attempting to destroy a locked mutex.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/gt/intel_context.c | 330 +++++++++++++-----------
>   drivers/gpu/drm/i915/i915_active.c      |  10 +-
>   drivers/gpu/drm/i915/i915_active.h      |   1 +
>   3 files changed, 192 insertions(+), 149 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index 52db2bde44a3..cb36cc26f95a 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -22,7 +22,7 @@ static struct i915_global_context {
>   
>   static struct intel_context *intel_context_alloc(void)
>   {
> -	return kmem_cache_zalloc(global.slab_ce, GFP_KERNEL);
> +	return kmem_cache_alloc(global.slab_ce, GFP_KERNEL);
>   }
>   
>   void intel_context_free(struct intel_context *ce)
> @@ -30,6 +30,176 @@ void intel_context_free(struct intel_context *ce)
>   	kmem_cache_free(global.slab_ce, ce);
>   }
>   
> +static int __context_pin_state(struct i915_vma *vma)
> +{
> +	unsigned int bias = i915_ggtt_pin_bias(vma) | PIN_OFFSET_BIAS;
> +	int err;
> +
> +	err = i915_ggtt_pin(vma, 0, bias | PIN_HIGH);
> +	if (err)
> +		return err;
> +
> +	err = i915_active_acquire(&vma->active);
> +	if (err)
> +		goto err_unpin;
> +
> +	/*
> +	 * And mark it as a globally pinned object to let the shrinker know
> +	 * it cannot reclaim the object until we release it.
> +	 */
> +	i915_vma_make_unshrinkable(vma);
> +	vma->obj->mm.dirty = true;
> +
> +	return 0;
> +
> +err_unpin:
> +	i915_vma_unpin(vma);
> +	return err;
> +}
> +
> +static void __context_unpin_state(struct i915_vma *vma)
> +{
> +	i915_vma_make_shrinkable(vma);
> +	i915_active_release(&vma->active);
> +	__i915_vma_unpin(vma);
> +}
> +
> +static int __ring_active(struct intel_ring *ring)
> +{
> +	int err;
> +
> +	err = intel_ring_pin(ring);
> +	if (err)
> +		return err;
> +
> +	err = i915_active_acquire(&ring->vma->active);
> +	if (err)
> +		goto err_pin;
> +
> +	return 0;
> +
> +err_pin:
> +	intel_ring_unpin(ring);
> +	return err;
> +}
> +
> +static void __ring_retire(struct intel_ring *ring)
> +{
> +	i915_active_release(&ring->vma->active);
> +	intel_ring_unpin(ring);
> +}
> +
> +__i915_active_call
> +static void __intel_context_retire(struct i915_active *active)
> +{
> +	struct intel_context *ce = container_of(active, typeof(*ce), active);
> +
> +	CE_TRACE(ce, "retire runtime: { total:%lluns, avg:%lluns }\n",
> +		 intel_context_get_total_runtime_ns(ce),
> +		 intel_context_get_avg_runtime_ns(ce));
> +
> +	set_bit(CONTEXT_VALID_BIT, &ce->flags);
> +	if (ce->state)
> +		__context_unpin_state(ce->state);
> +
> +	intel_timeline_unpin(ce->timeline);
> +	__ring_retire(ce->ring);
> +
> +	intel_context_put(ce);
> +}
> +
> +static int __intel_context_active(struct i915_active *active)
> +{
> +	struct intel_context *ce = container_of(active, typeof(*ce), active);
> +	int err;
> +
> +	CE_TRACE(ce, "active\n");
> +
> +	intel_context_get(ce);
> +
> +	err = __ring_active(ce->ring);
> +	if (err)
> +		goto err_put;
> +
> +	err = intel_timeline_pin(ce->timeline);
> +	if (err)
> +		goto err_ring;
> +
> +	if (!ce->state)
> +		return 0;
> +
> +	err = __context_pin_state(ce->state);
> +	if (err)
> +		goto err_timeline;
> +
> +	return 0;
> +
> +err_timeline:
> +	intel_timeline_unpin(ce->timeline);
> +err_ring:
> +	__ring_retire(ce->ring);
> +err_put:
> +	intel_context_put(ce);
> +	return err;
> +}
> +
> +static void __intel_context_ctor(void *arg)
> +{
> +	struct intel_context *ce = arg;
> +
> +	INIT_LIST_HEAD(&ce->signal_link);
> +	INIT_LIST_HEAD(&ce->signals);
> +
> +	atomic_set(&ce->pin_count, 0);
> +	mutex_init(&ce->pin_mutex);
> +
> +	ce->active_count = 0;
> +	i915_active_init(&ce->active,
> +			 __intel_context_active, __intel_context_retire);
> +
> +	ce->inflight = NULL;
> +	ce->lrc_reg_state = NULL;
> +	ce->lrc.desc = 0;
> +}
> +
> +static void
> +__intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
> +{
> +	GEM_BUG_ON(!engine->cops);
> +	GEM_BUG_ON(!engine->gt->vm);
> +
> +	kref_init(&ce->ref);
> +	i915_active_reinit(&ce->active);
> +
> +	ce->engine = engine;
> +	ce->ops = engine->cops;
> +	ce->sseu = engine->sseu;
> +
> +	ce->wa_bb_page = 0;
> +	ce->flags = 0;
> +	ce->tag = 0;
> +
> +	memset(&ce->runtime, 0, sizeof(ce->runtime));
> +
> +	ce->vm = i915_vm_get(engine->gt->vm);
> +	ce->gem_context = NULL;
> +
> +	ce->ring = __intel_context_ring_size(SZ_4K);
> +	ce->timeline = NULL;
> +	ce->state = NULL;
> +
> +	GEM_BUG_ON(atomic_read(&ce->pin_count));
> +	GEM_BUG_ON(ce->active_count);
> +	GEM_BUG_ON(ce->inflight);
> +}
> +
> +void
> +intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
> +{
> +	__intel_context_ctor(ce);
> +	__intel_context_init(ce, engine);

Which bits are accessed from outside context free (inside the RCU 
period) and based on which ones is the decision taken in the caller that 
the context is free so should be skipped?

And arent' both _ctor and _init fields re-initialized in the same go 
with this approach (no RCU period between them) - that is - object can 
get recycled instantly so what is the point in this case between the 
_ctor and _init split?

Regards,

Tvrtko

> +}
> +
>   struct intel_context *
>   intel_context_create(struct intel_engine_cs *engine)
>   {
> @@ -39,7 +209,7 @@ intel_context_create(struct intel_engine_cs *engine)
>   	if (!ce)
>   		return ERR_PTR(-ENOMEM);
>   
> -	intel_context_init(ce, engine);
> +	__intel_context_init(ce, engine);
>   	return ce;
>   }
>   
> @@ -158,161 +328,19 @@ void intel_context_unpin(struct intel_context *ce)
>   	/*
>   	 * Once released, we may asynchronously drop the active reference.
>   	 * As that may be the only reference keeping the context alive,
> -	 * take an extra now so that it is not freed before we finish
> +	 * hold onto RCU so that it is not freed before we finish
>   	 * dereferencing it.
>   	 */
> -	intel_context_get(ce);
> +	rcu_read_lock();
>   	intel_context_active_release(ce);
> -	intel_context_put(ce);
> -}
> -
> -static int __context_pin_state(struct i915_vma *vma)
> -{
> -	unsigned int bias = i915_ggtt_pin_bias(vma) | PIN_OFFSET_BIAS;
> -	int err;
> -
> -	err = i915_ggtt_pin(vma, 0, bias | PIN_HIGH);
> -	if (err)
> -		return err;
> -
> -	err = i915_active_acquire(&vma->active);
> -	if (err)
> -		goto err_unpin;
> -
> -	/*
> -	 * And mark it as a globally pinned object to let the shrinker know
> -	 * it cannot reclaim the object until we release it.
> -	 */
> -	i915_vma_make_unshrinkable(vma);
> -	vma->obj->mm.dirty = true;
> -
> -	return 0;
> -
> -err_unpin:
> -	i915_vma_unpin(vma);
> -	return err;
> -}
> -
> -static void __context_unpin_state(struct i915_vma *vma)
> -{
> -	i915_vma_make_shrinkable(vma);
> -	i915_active_release(&vma->active);
> -	__i915_vma_unpin(vma);
> -}
> -
> -static int __ring_active(struct intel_ring *ring)
> -{
> -	int err;
> -
> -	err = intel_ring_pin(ring);
> -	if (err)
> -		return err;
> -
> -	err = i915_active_acquire(&ring->vma->active);
> -	if (err)
> -		goto err_pin;
> -
> -	return 0;
> -
> -err_pin:
> -	intel_ring_unpin(ring);
> -	return err;
> -}
> -
> -static void __ring_retire(struct intel_ring *ring)
> -{
> -	i915_active_release(&ring->vma->active);
> -	intel_ring_unpin(ring);
> +	rcu_read_unlock();
>   }
> -
> -__i915_active_call
> -static void __intel_context_retire(struct i915_active *active)
> -{
> -	struct intel_context *ce = container_of(active, typeof(*ce), active);
> -
> -	CE_TRACE(ce, "retire runtime: { total:%lluns, avg:%lluns }\n",
> -		 intel_context_get_total_runtime_ns(ce),
> -		 intel_context_get_avg_runtime_ns(ce));
> -
> -	set_bit(CONTEXT_VALID_BIT, &ce->flags);
> -	if (ce->state)
> -		__context_unpin_state(ce->state);
> -
> -	intel_timeline_unpin(ce->timeline);
> -	__ring_retire(ce->ring);
> -
> -	intel_context_put(ce);
> -}
> -
> -static int __intel_context_active(struct i915_active *active)
> -{
> -	struct intel_context *ce = container_of(active, typeof(*ce), active);
> -	int err;
> -
> -	CE_TRACE(ce, "active\n");
> -
> -	intel_context_get(ce);
> -
> -	err = __ring_active(ce->ring);
> -	if (err)
> -		goto err_put;
> -
> -	err = intel_timeline_pin(ce->timeline);
> -	if (err)
> -		goto err_ring;
> -
> -	if (!ce->state)
> -		return 0;
> -
> -	err = __context_pin_state(ce->state);
> -	if (err)
> -		goto err_timeline;
> -
> -	return 0;
> -
> -err_timeline:
> -	intel_timeline_unpin(ce->timeline);
> -err_ring:
> -	__ring_retire(ce->ring);
> -err_put:
> -	intel_context_put(ce);
> -	return err;
> -}
> -
> -void
> -intel_context_init(struct intel_context *ce,
> -		   struct intel_engine_cs *engine)
> -{
> -	GEM_BUG_ON(!engine->cops);
> -	GEM_BUG_ON(!engine->gt->vm);
> -
> -	kref_init(&ce->ref);
> -
> -	ce->engine = engine;
> -	ce->ops = engine->cops;
> -	ce->sseu = engine->sseu;
> -	ce->ring = __intel_context_ring_size(SZ_4K);
> -
> -	ewma_runtime_init(&ce->runtime.avg);
> -
> -	ce->vm = i915_vm_get(engine->gt->vm);
> -
> -	INIT_LIST_HEAD(&ce->signal_link);
> -	INIT_LIST_HEAD(&ce->signals);
> -
> -	mutex_init(&ce->pin_mutex);
> -
> -	i915_active_init(&ce->active,
> -			 __intel_context_active, __intel_context_retire);
> -}
> -
>   void intel_context_fini(struct intel_context *ce)
>   {
>   	if (ce->timeline)
>   		intel_timeline_put(ce->timeline);
>   	i915_vm_put(ce->vm);
>   
> -	mutex_destroy(&ce->pin_mutex);
>   	i915_active_fini(&ce->active);
>   }
>   
> @@ -333,7 +361,13 @@ static struct i915_global_context global = { {
>   
>   int __init i915_global_context_init(void)
>   {
> -	global.slab_ce = KMEM_CACHE(intel_context, SLAB_HWCACHE_ALIGN);
> +	global.slab_ce =
> +		kmem_cache_create("intel_context",
> +				  sizeof(struct intel_context),
> +				  __alignof__(struct intel_context),
> +				  SLAB_HWCACHE_ALIGN |
> +				  SLAB_TYPESAFE_BY_RCU,
> +				  __intel_context_ctor);
>   	if (!global.slab_ce)
>   		return -ENOMEM;
>   
> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> index b0a6522be3d1..d25e60d8e91c 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -789,12 +789,20 @@ void i915_active_fini(struct i915_active *ref)
>   	debug_active_fini(ref);
>   	GEM_BUG_ON(atomic_read(&ref->count));
>   	GEM_BUG_ON(work_pending(&ref->work));
> -	mutex_destroy(&ref->mutex);
>   
>   	if (ref->cache)
>   		kmem_cache_free(global.slab_cache, ref->cache);
>   }
>   
> +void i915_active_reinit(struct i915_active *ref)
> +{
> +	GEM_BUG_ON(!i915_active_is_idle(ref));
> +	debug_active_init(ref);
> +
> +	ref->cache = NULL;
> +	ref->tree = RB_ROOT;
> +}
> +
>   static inline bool is_idle_barrier(struct active_node *node, u64 idx)
>   {
>   	return node->timeline == idx && !i915_active_fence_isset(&node->base);
> diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
> index fb165d3f01cf..bebab58ef4fe 100644
> --- a/drivers/gpu/drm/i915/i915_active.h
> +++ b/drivers/gpu/drm/i915/i915_active.h
> @@ -218,6 +218,7 @@ i915_active_is_idle(const struct i915_active *ref)
>   }
>   
>   void i915_active_fini(struct i915_active *ref);
> +void i915_active_reinit(struct i915_active *ref);
>   
>   int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
>   					    struct intel_engine_cs *engine);
> 
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