[Intel-gfx] [PATCH 14/15] drm/i915: Disable pagefaults along execbuffer relocation fast path

[Chris Wilson]

Along the fast path for relocation handling, we attempt to copy directly
from the user data structures whilst holding our mutex. This causes
lockdep to warn about circular lock dependencies if we need to pagefault
the user pages. [Since when handling a page fault on a mmapped bo, we
need to acquire the struct mutex whilst already holding the mm
semaphore, it is then verboten to acquire the mm semaphore when already
holding the struct mutex. The likelihood of the user passing in the
relocations contained in a GTT mmaped bo is low, but conceivable for
extreme pathology.] In order to force the mm to return EFAULT rather
than handle the pagefault, we therefore need to disable pagefaults
across the relocation fast path.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: stable at kernel.org
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 8367fc9..f98a213 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -441,13 +441,15 @@ i915_gem_execbuffer_relocate(struct drm_device *dev,
 	struct drm_i915_gem_object *obj;
 	int ret;
 
+	pagefault_disable();
 	list_for_each_entry(obj, objects, exec_list) {
 		ret = i915_gem_execbuffer_relocate_object(obj, eb);
 		if (ret)
-			return ret;
+			break;
 	}
+	pagefault_enable();
 
-	return 0;
+	return ret;
 }
 
 static int
-- 
1.7.4.1