The intel_ring.head is updated as the requests are retired, but is
sampled at any time as we submit requests. Furthermore, it tracks
RING_HEAD which is inherently asynchronous.

[  148.630314] BUG: KCSAN: data-race in execlists_dequeue [i915] / i915_request_retire [i915]
[  148.630349]
[  148.630374] write to 0xffff8881f4e28ddc of 4 bytes by task 90 on cpu 2:
[  148.630752]  i915_request_retire+0xed/0x770 [i915]
[  148.631123]  retire_requests+0x7a/0xd0 [i915]
[  148.631491]  engine_retire+0xa6/0xe0 [i915]
[  148.631523]  process_one_work+0x3af/0x640
[  148.631552]  worker_thread+0x80/0x670
[  148.631581]  kthread+0x19a/0x1e0
[  148.631609]  ret_from_fork+0x1f/0x30
[  148.631629]
[  148.631652] read to 0xffff8881f4e28ddc of 4 bytes by task 14288 on cpu 3:
[  148.632019]  execlists_dequeue+0x1300/0x1680 [i915]
[  148.632384]  __execlists_submission_tasklet+0x48/0x60 [i915]
[  148.632770]  execlists_submit_request+0x38e/0x3c0 [i915]
[  148.633146]  submit_notify+0x8f/0xc0 [i915]
[  148.633512]  __i915_sw_fence_complete+0x5d/0x3e0 [i915]
[  148.633875]  i915_sw_fence_complete+0x58/0x80 [i915]
[  148.634238]  i915_sw_fence_commit+0x16/0x20 [i915]
[  148.634613]  __i915_request_queue+0x60/0x70 [i915]
[  148.634985]  i915_gem_do_execbuffer+0x2de0/0x42b0 [i915]
[  148.635366]  i915_gem_execbuffer2_ioctl+0x2ab/0x580 [i915]
[  148.635400]  drm_ioctl_kernel+0xe9/0x130
[  148.635429]  drm_ioctl+0x27d/0x45e
[  148.635456]  ksys_ioctl+0x89/0xb0
[  148.635482]  __x64_sys_ioctl+0x42/0x60
[  148.635510]  do_syscall_64+0x6e/0x2c0
[  148.635542]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

[  645.071436] BUG: KCSAN: data-race in gen8_emit_fini_breadcrumb [i915] / i915_request_retire [i915]
[  645.071456]
[  645.071467] write to 0xffff8881efe403dc of 4 bytes by task 14668 on cpu 3:
[  645.071647]  i915_request_retire+0xed/0x770 [i915]
[  645.071824]  i915_request_create+0x6c/0x160 [i915]
[  645.072000]  i915_gem_do_execbuffer+0x206d/0x42b0 [i915]
[  645.072177]  i915_gem_execbuffer2_ioctl+0x2ab/0x580 [i915]
[  645.072194]  drm_ioctl_kernel+0xe9/0x130
[  645.072208]  drm_ioctl+0x27d/0x45e
[  645.072222]  ksys_ioctl+0x89/0xb0
[  645.072235]  __x64_sys_ioctl+0x42/0x60
[  645.072248]  do_syscall_64+0x6e/0x2c0
[  645.072263]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  645.072275]
[  645.072285] read to 0xffff8881efe403dc of 4 bytes by interrupt on cpu 2:
[  645.072458]  gen8_emit_fini_breadcrumb+0x158/0x300 [i915]
[  645.072636]  __i915_request_submit+0x204/0x430 [i915]
[  645.072809]  execlists_dequeue+0x8e1/0x1680 [i915]
[  645.072982]  __execlists_submission_tasklet+0x48/0x60 [i915]
[  645.073154]  execlists_submit_request+0x38e/0x3c0 [i915]
[  645.073330]  submit_notify+0x8f/0xc0 [i915]
[  645.073499]  __i915_sw_fence_complete+0x5d/0x3e0 [i915]
[  645.073668]  i915_sw_fence_wake+0xc2/0x130 [i915]
[  645.073836]  __i915_sw_fence_complete+0x2cf/0x3e0 [i915]
[  645.074006]  i915_sw_fence_complete+0x58/0x80 [i915]
[  645.074175]  dma_i915_sw_fence_wake+0x3e/0x80 [i915]
[  645.074344]  signal_irq_work+0x62f/0x710 [i915]
[  645.074360]  irq_work_run_list+0xd7/0x110
[  645.074373]  irq_work_run+0x1d/0x50
[  645.074386]  smp_irq_work_interrupt+0x21/0x30
[  645.074400]  irq_work_interrupt+0xf/0x20
[  645.074414]  _raw_spin_unlock_irqrestore+0x34/0x40
[  645.074585]  execlists_submission_tasklet+0xde/0x170 [i915]
[  645.074602]  tasklet_action_common.isra.0+0x42/0x90
[  645.074617]  __do_softirq+0xc8/0x206
[  645.074629]  irq_exit+0xcd/0xe0
[  645.074642]  do_IRQ+0x44/0xc0
[  645.074654]  ret_from_intr+0x0/0x1c
[  645.074667]  finish_task_switch+0x73/0x230
[  645.074679]  __schedule+0x1c5/0x4c0
[  645.074691]  schedule+0x45/0xb0
[  645.074704]  worker_thread+0x194/0x670
[  645.074716]  kthread+0x19a/0x1e0
[  645.074729]  ret_from_fork+0x1f/0x30
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200407221832.15465-1-chris@chris-wilson.co.uk