drm/sched: Check scheduler work queue before calling timeout handling

In the Linux kernel, the following vulnerability has been resolved:

drm/sched: Check scheduler work queue before calling timeout handling

During an IGT GPU reset test we see again oops despite of

commit 0c8c901aaaebc9 (drm/sched: Check scheduler ready before calling

timeout handling).

It uses ready condition whether to call drm_sched_fault which unwind

the TDR leads to GPU reset.

However it looks the ready condition is overloaded with other meanings,

for example, for the following stack is related GPU reset :

0 gfx_v9_0_cp_gfx_start

1 gfx_v9_0_cp_gfx_resume

2 gfx_v9_0_cp_resume

3 gfx_v9_0_hw_init

4 gfx_v9_0_resume

5 amdgpu_device_ip_resume_phase2

does the following:

/* start the ring */

gfx_v9_0_cp_gfx_start(adev);

ring->sched.ready = true;

The same approach is for other ASICs as well :

gfx_v8_0_cp_gfx_resume

gfx_v10_0_kiq_resume, etc...

As a result, our GPU reset test causes GPU fault which calls unconditionally gfx_v9_0_fault

and then drm_sched_fault. However now it depends on whether the interrupt service routine

drm_sched_fault is executed after gfx_v9_0_cp_gfx_start is completed which sets the ready

field of the scheduler to true even for uninitialized schedulers and causes oops vs

no fault or when ISR drm_sched_fault is completed prior gfx_v9_0_cp_gfx_start and

NULL pointer dereference does not occur.

Use the field timeout_wq to prevent oops for uninitialized schedulers.

The field could be initialized by the work queue of resetting the domain.

v1: Corrections to commit message (Luben)