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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().

Leo Li committed 8 years ago

Commit 4364bcb1 parent 9068e023 files · +24 −4Browse files

Unified · Split

3 files
  1. drivers/gpu/drm/drm_atomic.c +5 −0
  2. drivers/gpu/drm/drm_atomic_helper.c +8 −4
  3. include/drm/drm_atomic.h +11 −0
drivers/gpu/drm/drm_atomic.c+5−0
@@ -174,6 +174,11 @@ void drm_atomic_state_default_clear(struct drm_atomic_state *state) · 173 unmodified lines
174174 state->crtcs[i].state = NULL;
175175 state->crtcs[i].old_state = NULL;
176176 state->crtcs[i].new_state = NULL;
177+
178+ if (state->crtcs[i].commit) {
179+ drm_crtc_commit_put(state->crtcs[i].commit);
180+ state->crtcs[i].commit = NULL;
181+ }
177182 }
178183
179184 for (i = 0; i < config->num_total_plane; i++) {
drivers/gpu/drm/drm_atomic_helper.c+8−4
@@ -1408,15 +1408,16 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks); · 1407 unmodified lines
14081408 void drm_atomic_helper_wait_for_flip_done(struct drm_device *dev,
14091409 struct drm_atomic_state *old_state)
14101410 {
1411− struct drm_crtc_state *new_crtc_state;
14121411 struct drm_crtc *crtc;
14131412 int i;
14141413
1415− for_each_new_crtc_in_state(old_state, crtc, new_crtc_state, i) {
1416− struct drm_crtc_commit *commit = new_crtc_state->commit;
1414+ for (i = 0; i < dev->mode_config.num_crtc; i++) {
1415+ struct drm_crtc_commit *commit = old_state->crtcs[i].commit;
14171416 int ret;
14181417
1419− if (!commit)
1418+ crtc = old_state->crtcs[i].ptr;
1419+
1420+ if (!crtc || !commit)
14201421 continue;
14211422
14221423 ret = wait_for_completion_timeout(&commit->flip_done, 10 * HZ);
@@ -1934,6 +1935,9 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state, · 511 unmodified lines
19341935 drm_crtc_commit_get(commit);
19351936
19361937 commit->abort_completion = true;
1938+
1939+ state->crtcs[i].commit = commit;
1940+ drm_crtc_commit_get(commit);
19371941 }
19381942
19391943 for_each_oldnew_connector_in_state(state, conn, old_conn_state, new_conn_state, i) {
include/drm/drm_atomic.h+11−0
@@ -153,6 +153,17 @@ struct __drm_planes_state { · 152 unmodified lines
153153 struct __drm_crtcs_state {
154154 struct drm_crtc *ptr;
155155 struct drm_crtc_state *state, *old_state, *new_state;
156+
157+ /**
158+ * @commit:
159+ *
160+ * A reference to the CRTC commit object that is kept for use by
161+ * drm_atomic_helper_wait_for_flip_done() after
162+ * drm_atomic_helper_commit_hw_done() is called. This ensures that a
163+ * concurrent commit won't free a commit object that is still in use.
164+ */
165+ struct drm_crtc_commit *commit;
166+
156167 s32 __user *out_fence_ptr;
157168 u64 last_vblank_count;
158169 };