Re: [Bug #12491] i915 lockdep warning

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

[intel-gfx CC added]

 > I tested this on my 965 based system that showed the issue and it seemed resolved, at least no warnings in dmesg.  Lockdep was confirmed to still be enabled.

 > I can mark the bug fixed in korg bugzilla too.

Probably we just want to add a comment saying the patch works -- the bug
should stay open until we get the fix upstream I guess.

 > Thank you Roland for looking into this, good work!

Thanks for testing!  Patch with updated changelog (including your
tested-by) below.  Jesse [Barnes] and/or Eric please review and apply:

---

i915: Fix potential AB-BA deadlock in i915_gem_execbuffer()

Lockdep warns that i915_gem_execbuffer() can trigger a page fault (which
takes mmap_sem) while holding dev->struct_mutex, while drm_vm_open()
(which is called with mmap_sem already held) takes dev->struct_mutex.
So this is a potential AB-BA deadlock.

The way that i915_gem_execbuffer() triggers a page fault is by doing
copy_to_user() when returning new buffer offsets back to userspace;
however there is no reason to hold the struct_mutex when doing this
copy, since what is being copied is the contents of an array private to
i915_gem_execbuffer() anyway.  So we can fix the potential deadlock (and
get rid of the lockdep warning) by simply moving the copy_to_user()
outside of where struct_mutex is held.

This fixes <http://bugzilla.kernel.org/show_bug.cgi?id=12491>.

Reported-by: Jesse Brandeburg <jesse.brandeburg-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Tested-by: Jesse Brandeburg <jesse.brandeburg-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Signed-off-by: Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>
---
 drivers/gpu/drm/i915/i915_gem.c |   21 ++++++++++++---------
 1 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index debad5c..23aad8c 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2610,15 +2610,6 @@ i915_gem_execbuffer(struct drm_device *dev, void *data,
 
 	i915_verify_inactive(dev, __FILE__, __LINE__);
 
-	/* Copy the new buffer offsets back to the user's exec list. */
-	ret = copy_to_user((struct drm_i915_relocation_entry __user *)
-			   (uintptr_t) args->buffers_ptr,
-			   exec_list,
-			   sizeof(*exec_list) * args->buffer_count);
-	if (ret)
-		DRM_ERROR("failed to copy %d exec entries "
-			  "back to user (%d)\n",
-			   args->buffer_count, ret);
 err:
 	for (i = 0; i < pinned; i++)
 		i915_gem_object_unpin(object_list[i]);
@@ -2628,6 +2619,18 @@ err:
 
 	mutex_unlock(&dev->struct_mutex);
 
+	if (!ret) {
+		/* Copy the new buffer offsets back to the user's exec list. */
+		ret = copy_to_user((struct drm_i915_relocation_entry __user *)
+				   (uintptr_t) args->buffers_ptr,
+				   exec_list,
+				   sizeof(*exec_list) * args->buffer_count);
+		if (ret)
+			DRM_ERROR("failed to copy %d exec entries "
+				  "back to user (%d)\n",
+				  args->buffer_count, ret);
+	}
+
 pre_mutex_err:
 	drm_free(object_list, sizeof(*object_list) * args->buffer_count,
 		 DRM_MEM_DRIVER);