[PATCH v2 2/2] drm/drm_crtc: fix race with dma_fence_signal() in ::get_driver_name()

["André Draszik" <andre.draszik@linaro.org>]

Since commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"),
I'm seeing the BUG_ON() triggering in drm_crtc's fence_to_crtc() via
drm_crtc_fence_get_driver_name() regularly:

    Call trace:
     panic+0x58/0x5c
     die+0x160/0x178
     bug_brk_handler+0x70/0xa4
     call_el1_break_hook+0x3c/0x1a0
     do_el1_brk64+0x24/0x74
     el1_brk64+0x34/0x54
     el1h_64_sync_handler+0x80/0xfc
     el1h_64_sync+0x84/0x88
     drm_crtc_fence_get_driver_name+0x60/0x68 (P)
     sync_file_get_name+0x184/0x45c
     sync_file_ioctl+0x404/0xf70
     __arm64_sys_ioctl+0x124/0x1dc

This looks to be caused by a code flow similar to the following:

+++ snip +++
thread A                             thread B

                                     ioctl(SYNC_IOC_FILE_INFO)
                                     sync_file_ioctl()
                                     sync_file_get_name()
dma_fence_signal_timestamp_locked()  dma_fence_driver_name()
                                       ops = rcu_dereference(fence->ops)
                                       if (!dma_fence_test_signaled_flag())
                                         ops->get_driver_name(fence) i.e.
                                         drm_crtc_fence_get_driver_name()
test_and_set_bit(SIGNALED)
RCU_INIT_POINTER(fence->ops, NULL)
                                     drm_crtc_fence_get_driver_name()
                                       BUG_ON(rcu_access_pointer(fence->ops)
                                              != &drm_crtc_fence_ops)
+++ snap +++

I see two ways to resolve this:
a) simply drop the BUG_ON(). It can not work anymore since above
   commit, as it is racy now.
b) pass the original 'ops' pointer obtained in dma_fence_driver_name()
   to all callees.

This patch implements option a), as because:
* I don't see much benefit in passing the extra pointer just for this
  BUG_ON() to work.
* Requiring the dma_fence_ops in those callbacks is an implementation
  detail of the drm_crtc driver, and therefore upper layers shouldn't
  have to care about that.
* The existence of the BUG_ON() doesn't appear to be consistent with
  implementations of ::get_driver_name() or ::get_timeline_name() in
  the majority of other DRM drivers in the first place. Those that do
  have a similar BUG_ON() (i915, xe) probably also need an update
  similar to this patch here but I'm not in a position to test those.
* Using BUG() and friends to take down the system is an unacceptable
  way to handle a failure as evidenced by many threads on LKML and also
  in the kernel coding style. Here, the check was presumably added for
  detecting when something passes an invalid pointer, but that does not
  happen - and if it could, gracefully handling that situation would be
  more appropriate.

Note that the adjacent drm_crtc_fence_get_timeline_name() has the same
problem and is fixed by this patch as well.

Fixes: 541c8f2468b9 ("dma-buf: detach fence ops on signal v3")
Signed-off-by: André Draszik <andre.draszik@linaro.org>

---
v2:
- don't turn fence_to_crtc() into macro
- update commit message to include reference to unacceptable use of BUG
---
 drivers/gpu/drm/drm_crtc.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index d55f1377ec36..36ae50ddf525 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -154,11 +154,8 @@ static void drm_crtc_crc_fini(struct drm_crtc *crtc)
 #endif
 }
 
-static const struct dma_fence_ops drm_crtc_fence_ops;
-
 static struct drm_crtc *fence_to_crtc(struct dma_fence *fence)
 {
-	BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops);
 	return container_of(fence->extern_lock, struct drm_crtc, fence_lock);
 }
 

-- 
2.55.0.795.g602f6c329a-goog