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

[Chris Wilson]

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.
v3: As the intended use will remain strongly referenced counted, with
very little inflight access across reuse, drop the ctor.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_context.c       | 27 ++++++++++++++-----
 drivers/gpu/drm/i915/gt/intel_context_types.h |  6 +++++
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index 52db2bde44a3..8eebb31c6c28 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)
@@ -158,12 +158,12 @@ 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);
+	rcu_read_unlock();
 }
 
 static int __context_pin_state(struct i915_vma *vma)
@@ -280,8 +280,7 @@ static int __intel_context_active(struct i915_active *active)
 }
 
 void
-intel_context_init(struct intel_context *ce,
-		   struct intel_engine_cs *engine)
+intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
 {
 	GEM_BUG_ON(!engine->cops);
 	GEM_BUG_ON(!engine->gt->vm);
@@ -293,6 +292,12 @@ intel_context_init(struct intel_context *ce,
 	ce->sseu = engine->sseu;
 	ce->ring = __intel_context_ring_size(SZ_4K);
 
+	ce->wa_bb_page = 0;
+	ce->flags = 0;
+	ce->tag = 0;
+
+	memset(&ce->runtime, 0, sizeof(ce->runtime));
+
 	ewma_runtime_init(&ce->runtime.avg);
 
 	ce->vm = i915_vm_get(engine->gt->vm);
@@ -300,10 +305,16 @@ intel_context_init(struct intel_context *ce,
 	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;
 }
 
 void intel_context_fini(struct intel_context *ce)
@@ -333,7 +344,9 @@ 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(intel_context,
+				    SLAB_HWCACHE_ALIGN |
+				    SLAB_TYPESAFE_BY_RCU);
 	if (!global.slab_ce)
 		return -ENOMEM;
 
diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
index 4954b0df4864..18622f1a0249 100644
--- a/drivers/gpu/drm/i915/gt/intel_context_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
@@ -41,6 +41,12 @@ struct intel_context_ops {
 };
 
 struct intel_context {
+	/*
+	 * Note: Some fields may be accessed under RCU.
+	 *
+	 * Unless otherwise noted a field can safely be assumed to be protected
+	 * by strong reference counting.
+	 */
 	struct kref ref;
 
 	struct intel_engine_cs *engine;
-- 
2.20.1