drm/i915: Reduce nested prepare_remote_context() to a trylock

On context retiring, we may invoke the kernel_context to unpin this
context. Elsewhere, we may use the kernel_context to modify this
context. This currently leads to an AB-BA lock inversion, so we need to
back-off from the contended lock, and repeat.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111732
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: a9877da2d629 ("drm/i915/oa: Reconfigure contexts on the fly")
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126065521.2331017-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index ee9d2bcd2c13d5b2641a2a2668f70dfd8bf66f3a..ef7bc41ffffad537b981181129bb9772d3dc36fb 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -310,10 +310,23 @@ int intel_context_prepare_remote_request(struct intel_context *ce,
        GEM_BUG_ON(rq->hw_context == ce);
 
        if (rcu_access_pointer(rq->timeline) != tl) { /* timeline sharing! */
-               err = mutex_lock_interruptible_nested(&tl->mutex,
-                                                     SINGLE_DEPTH_NESTING);
-               if (err)
-                       return err;
+               /*
+                * Ideally, we just want to insert our foreign fence as
+                * a barrier into the remove context, such that this operation
+                * occurs after all current operations in that context, and
+                * all future operations must occur after this.
+                *
+                * Currently, the timeline->last_request tracking is guarded
+                * by its mutex and so we must obtain that to atomically
+                * insert our barrier. However, since we already hold our
+                * timeline->mutex, we must be careful against potential
+                * inversion if we are the kernel_context as the remote context
+                * will itself poke at the kernel_context when it needs to
+                * unpin. Ergo, if already locked, we drop both locks and
+                * try again (through the magic of userspace repeating EAGAIN).
+                */
+               if (!mutex_trylock(&tl->mutex))
+                       return -EAGAIN;
 
                /* Queue this switch after current activity by this context. */
                err = i915_active_fence_set(&tl->last_request, rq);