drm: Get ref on CRTC commit object when waiting for flip_done

This fixes a general protection fault, caused by accessing the contents
of a flip_done completion object that has already been freed. It occurs
due to the preemption of a non-blocking commit worker thread W by
another commit thread X. X continues to clear its atomic state at the
end, destroying the CRTC commit object that W still needs. Switching
back to W and accessing the commit objects then leads to bad results.

Worker W becomes preemptable when waiting for flip_done to complete. At
this point, a frequently occurring commit thread X can take over. Here's
an example where W is a worker thread that flips on both CRTCs, and X
does a legacy cursor update on both CRTCs:

...
1. W does flip work
2. W runs commit_hw_done()
3. W waits for flip_done on CRTC 1
4. > flip_done for CRTC 1 completes
5. W finishes waiting for CRTC 1
6. W waits for flip_done on CRTC 2

7. > Preempted by X
8. > flip_done for CRTC 2 completes
9. X atomic_check: hw_done and flip_done are complete on all CRTCs
10. X updates cursor on both CRTCs
11. X destroys atomic state
12. X done

13. > Switch back to W
14. W waits for flip_done on CRTC 2
15. W raises general protection fault

The error looks like so:

general protection fault: 0000 [#1] PREEMPT SMP PTI
**snip**
Call Trace:
lock_acquire+0xa2/0x1b0
_raw_spin_lock_irq+0x39/0x70
wait_for_completion_timeout+0x31/0x130
drm_atomic_helper_wait_for_flip_done+0x64/0x90 [drm_kms_helper]
amdgpu_dm_atomic_commit_tail+0xcae/0xdd0 [amdgpu]
commit_tail+0x3d/0x70 [drm_kms_helper]
process_one_work+0x212/0x650
worker_thread+0x49/0x420
kthread+0xfb/0x130
ret_from_fork+0x3a/0x50
Modules linked in: x86_pkg_temp_thermal amdgpu(O) chash(O)
gpu_sched(O) drm_kms_helper(O) syscopyarea sysfillrect sysimgblt
fb_sys_fops ttm(O) drm(O)

Note that i915 has this issue masked, since hw_done is signaled after
waiting for flip_done. Doing so will block the cursor update from
happening until hw_done is signaled, preventing the cursor commit from
destroying the state.

v2: The reference on the commit object needs to be obtained before
hw_done() is signaled, since that's the point where another commit
is allowed to modify the state. Assuming that the
new_crtc_state->commit object still exists within flip_done() is
incorrect.

Fix by getting a reference in setup_commit(), and releasing it
during default_clear().

Signed-off-by: Leo Li <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Harry Wentland <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_atomic.c

···
 		state->crtcs[i].state = NULL;
 		state->crtcs[i].old_state = NULL;
 		state->crtcs[i].new_state = NULL;
+
+		if (state->crtcs[i].commit) {
+			drm_crtc_commit_put(state->crtcs[i].commit);
+			state->crtcs[i].commit = NULL;
+		}
 	}
 
 	for (i = 0; i < config->num_total_plane; i++) {

drivers/gpu/drm/drm_atomic_helper.c

···
 void drm_atomic_helper_wait_for_flip_done(struct drm_device *dev,
 					  struct drm_atomic_state *old_state)
 {
-	struct drm_crtc_state *new_crtc_state;
 	struct drm_crtc *crtc;
 	int i;
 
-	for_each_new_crtc_in_state(old_state, crtc, new_crtc_state, i) {
-		struct drm_crtc_commit *commit = new_crtc_state->commit;
+	for (i = 0; i < dev->mode_config.num_crtc; i++) {
+		struct drm_crtc_commit *commit = old_state->crtcs[i].commit;
 		int ret;
 
-		if (!commit)
+		crtc = old_state->crtcs[i].ptr;
+
+		if (!crtc || !commit)
 			continue;
 
 		ret = wait_for_completion_timeout(&commit->flip_done, 10 * HZ);
···
 		drm_crtc_commit_get(commit);
 
 		commit->abort_completion = true;
+
+		state->crtcs[i].commit = commit;
+		drm_crtc_commit_get(commit);
 	}
 
 	for_each_oldnew_connector_in_state(state, conn, old_conn_state, new_conn_state, i) {

include/drm/drm_atomic.h

···
 struct __drm_crtcs_state {
 	struct drm_crtc *ptr;
 	struct drm_crtc_state *state, *old_state, *new_state;
+
+	/**
+	 * @commit:
+	 *
+	 * A reference to the CRTC commit object that is kept for use by
+	 * drm_atomic_helper_wait_for_flip_done() after
+	 * drm_atomic_helper_commit_hw_done() is called. This ensures that a
+	 * concurrent commit won't free a commit object that is still in use.
+	 */
+	struct drm_crtc_commit *commit;
+
 	s32 __user *out_fence_ptr;
 	u64 last_vblank_count;
 };