Re: [PATCH v2] drm: Get ref on CRTC commit object when waiting for flip_done

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On 2018-10-16 08:33 AM, Daniel Vetter wrote:
> On Mon, Oct 15, 2018 at 09:46:40AM -0400, sunpeng.li@xxxxxxx wrote:
>> From: Leo Li <sunpeng.li@xxxxxxx>
>>
>> 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@xxxxxxx>
>> ---
>>
>> Sending again, this time to the correct mailing list :)
>>
>> Leo
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> 
> I think it'd be really good if you could get intel-gfx-ci to test this
> patch. Simply submit it to intel-gfx@xxxxxxxxxxxxxxxxxxxxx. I'll leave
> applying to one of the amd drm-misc committers, once it's passed CI.
> 
> 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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