drm/i915/gt: Guard timeline pinning without relying on struct_mutex

In preparation for removing struct_mutex from around context retirement,
we need to make timeline pinning and unpinning safe. Since multiple
engines/contexts can share a single timeline, we cannot rely on
borrowing the context mutex (otherwise we could state that the timeline
is only pinned/unpinned inside the context pin/unpin and so guarded by
it). However, we only perform a sequence of atomic operations inside the
timeline pin/unpin and the sequence of those operations is safe for a
concurrent unpin / pin, so we can relax the struct_mutex requirement.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190815205709.24285-3-chris@chris-wilson.co.uk

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/gt/mock_engine.c b/drivers/gpu/drm/i915/gt/mock_engine.c
index a63dd8a42cd4..54a11dde3076 100644
--- a/drivers/gpu/drm/i915/gt/mock_engine.c
+++ b/drivers/gpu/drm/i915/gt/mock_engine.c
@@ -34,13 +34,13 @@
 
 static void mock_timeline_pin(struct intel_timeline *tl)
 {
-	tl->pin_count++;
+	atomic_inc(&tl->pin_count);
 }
 
 static void mock_timeline_unpin(struct intel_timeline *tl)
 {
-	GEM_BUG_ON(!tl->pin_count);
-	tl->pin_count--;
+	GEM_BUG_ON(!atomic_read(&tl->pin_count));
+	atomic_dec(&tl->pin_count);
 }
 
 static struct intel_ring *mock_ring(struct intel_engine_cs *engine)