drm/i915: Coordinate i915_active with its own mutex

Forgo the struct_mutex serialisation for i915_active, and interpose its
own mutex handling for active/retire.

This is a multi-layered sleight-of-hand. First, we had to ensure that no
active/retire callbacks accidentally inverted the mutex ordering rules,
nor assumed that they were themselves serialised by struct_mutex. More
challenging though, is the rule over updating elements of the active
rbtree. Instead of the whole i915_active now being serialised by
struct_mutex, allocations/rotations of the tree are serialised by the
i915_active.mutex and individual nodes are serialised by the caller
using the i915_timeline.mutex (we need to use nested spinlocks to
interact with the dma_fence callback lists).

The pain point here is that instead of a single mutex around execbuf, we
now have to take a mutex for active tracker (one for each vma, context,
etc) and a couple of spinlocks for each fence update. The improvement in
fine grained locking allowing for multiple concurrent clients
(eventually!) should be worth it in typical loads.

v2: Add some comments that barely elucidate anything :(

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004134015.13204-6-chris@chris-wilson.co.uk

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index 6384a06aa5bf..a28ee754b7b4 100644
--- a/drivers/gpu/drm/i915/i915_gpu_error.c
+++ b/drivers/gpu/drm/i915/i915_gpu_error.c
@@ -1299,7 +1299,7 @@ capture_vma(struct capture_vma *next,
 	if (!c)
 		return next;
 
-	if (!i915_active_trygrab(&vma->active)) {
+	if (!i915_active_acquire_if_busy(&vma->active)) {
 		kfree(c);
 		return next;
 	}
@@ -1439,7 +1439,7 @@ gem_record_rings(struct i915_gpu_state *error, struct compress *compress)
 			*this->slot =
 				i915_error_object_create(i915, vma, compress);
 
-			i915_active_ungrab(&vma->active);
+			i915_active_release(&vma->active);
 			i915_vma_put(vma);
 
 			capture = this->next;