[PATCH] drm/i915: use xa_lock/unlock for fpriv->vm_xa lookups

[Daniel Vetter <daniel.vetter@ffwll.ch>]

We don't need the absolute speed of rcu for this. And
i915_address_space in general dont need rcu protection anywhere else,
after we've made gem contexts and engines a lot more immutable.

Note that this semantically reverts

commit aabbe344dc3ca5f7d8263a02608ba6179e8a4499
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Aug 30 19:03:25 2019 +0100

    drm/i915: Use RCU for unlocked vm_idr lookup

except we have the conversion from idr to xarray in between.

v2: kref_get_unless_zero is no longer required (Maarten)

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/gpu/drm/i915/i915_drv.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index be2392bbcecc..d89ff55d8fc8 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1874,11 +1874,11 @@ i915_gem_vm_lookup(struct drm_i915_file_private *file_priv, u32 id)
 {
 	struct i915_address_space *vm;
 
-	rcu_read_lock();
+	xa_lock(&file_priv->vm_xa);
 	vm = xa_load(&file_priv->vm_xa, id);
-	if (vm && !kref_get_unless_zero(&vm->ref))
-		vm = NULL;
-	rcu_read_unlock();
+	if (vm)
+		kref_get(&vm->ref);
+	xa_unlock(&file_priv->vm_xa);
 
 	return vm;
 }
-- 
2.33.0