drm/i915: Remove the vma refcount

Now that i915_vma_parked() is taking the object lock on vma destruction,
and the only user of the vma refcount, i915_gem_object_unbind()
also takes the object lock, remove the vma refcount.

v3: Documentation update.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304082641.308069-3-thomas.hellstrom@linux.intel.com

1 files changed, 13 insertions, 4 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index dd84ebabb50f..c26110abcc0b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -151,14 +151,25 @@ try_again:
 			break;
 		}
 
+		/*
+		 * Requiring the vm destructor to take the object lock
+		 * before destroying a vma would help us eliminate the
+		 * i915_vm_tryget() here, AND thus also the barrier stuff
+		 * at the end. That's an easy fix, but sleeping locks in
+		 * a kthread should generally be avoided.
+		 */
 		ret = -EAGAIN;
 		if (!i915_vm_tryget(vma->vm))
 			break;
 
-		/* Prevent vma being freed by i915_vma_parked as we unbind */
-		vma = __i915_vma_get(vma);
 		spin_unlock(&obj->vma.lock);
 
+		/*
+		 * Since i915_vma_parked() takes the object lock
+		 * before vma destruction, it won't race us here,
+		 * and destroy the vma from under us.
+		 */
+
 		if (vma) {
 			bool vm_trylock = !!(flags & I915_GEM_OBJECT_UNBIND_VM_TRYLOCK);
 			ret = -EBUSY;
@@ -180,8 +191,6 @@ try_again:
 					ret = i915_vma_unbind(vma);
 				}
 			}
-
-			__i915_vma_put(vma);
 		}
 
 		i915_vm_put(vma->vm);