[PATCH v2] drm: Get ref on CRTC commit object when waiting for flip_done
Alex Deucher
alexdeucher at gmail.com
Tue Oct 16 15:48:57 UTC 2018
On Tue, Oct 16, 2018 at 11:00 AM Li, Sun peng (Leo) <Sunpeng.Li at amd.com> wrote:
>
>
>
> On 2018-10-16 08:33 AM, Daniel Vetter wrote:
> > On Mon, Oct 15, 2018 at 09:46:40AM -0400, sunpeng.li at amd.com wrote:
> >> From: Leo Li <sunpeng.li at amd.com>
> >>
> >> 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 <sunpeng.li at amd.com>
> >> ---
> >>
> >> Sending again, this time to the correct mailing list :)
> >>
> >> Leo
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: stable at vger.kernel.org
> >
> > I think it'd be really good if you could get intel-gfx-ci to test this
> > patch. Simply submit it to intel-gfx at lists.freedesktop.org. I'll leave
> > applying to one of the amd drm-misc committers, once it's passed CI.
Leo, do you or Harry have drm-misc commit access yet? If not, you should.
Alex
> >
> > Cheers, Daniel
>
> Thanks for the rb!
>
> On the topic of CI, is it possible to write a test (maybe one already
> exists) for this issue? I've attempted to do one here:
>
> https://patchwork.freedesktop.org/patch/256319/
>
> The problem is finding a surefire way to trigger the sequence, I'm not
> sure how that can be done. If you have any ideas, I would love to hear them.
>
> Leo
>
> >
> >>
> >> drivers/gpu/drm/drm_atomic.c | 5 +++++
> >> drivers/gpu/drm/drm_atomic_helper.c | 12 ++++++++----
> >> include/drm/drm_atomic.h | 11 +++++++++++
> >> 3 files changed, 24 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> >> index 3eb061e..12ae917 100644
> >> --- a/drivers/gpu/drm/drm_atomic.c
> >> +++ b/drivers/gpu/drm/drm_atomic.c
> >> @@ -174,6 +174,11 @@ void drm_atomic_state_default_clear(struct drm_atomic_state *state)
> >> 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++) {
> >> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> >> index 80be74d..1bb4c31 100644
> >> --- a/drivers/gpu/drm/drm_atomic_helper.c
> >> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> >> @@ -1408,15 +1408,16 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_vblanks);
> >> 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);
> >> @@ -1934,6 +1935,9 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
> >> 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) {
> >> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> >> index da9d95a..1e71315 100644
> >> --- a/include/drm/drm_atomic.h
> >> +++ b/include/drm/drm_atomic.h
> >> @@ -153,6 +153,17 @@ struct __drm_planes_state {
> >> 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;
> >> };
> >> --
> >> 2.7.4
> >>
> >
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