drm/i915: Mark up unlocked update of i915_request.hwsp_seqno

During i915_request_retire() we decouple the i915_request.hwsp_seqno
from the intel_timeline so that it may be freed before the request is
released. However, we need to warn the compiler that the pointer may
update under its nose.

[  171.438899] BUG: KCSAN: data-race in i915_request_await_dma_fence [i915] / i915_request_retire [i915]
[  171.438920]
[  171.438932] write to 0xffff8881e7e28ce0 of 8 bytes by task 148 on cpu 2:
[  171.439174]  i915_request_retire+0x1ea/0x660 [i915]
[  171.439408]  retire_requests+0x7a/0xd0 [i915]
[  171.439640]  engine_retire+0xa1/0xe0 [i915]
[  171.439657]  process_one_work+0x3b1/0x690
[  171.439671]  worker_thread+0x80/0x670
[  171.439685]  kthread+0x19a/0x1e0
[  171.439701]  ret_from_fork+0x1f/0x30
[  171.439721]
[  171.439739] read to 0xffff8881e7e28ce0 of 8 bytes by task 696 on cpu 1:
[  171.439990]  i915_request_await_dma_fence+0x162/0x520 [i915]
[  171.440230]  i915_request_await_object+0x2fe/0x470 [i915]
[  171.440467]  i915_gem_do_execbuffer+0x45dc/0x4c20 [i915]
[  171.440704]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  171.440722]  drm_ioctl_kernel+0xe4/0x120
[  171.440736]  drm_ioctl+0x297/0x4c7
[  171.440750]  ksys_ioctl+0x89/0xb0
[  171.440766]  __x64_sys_ioctl+0x42/0x60
[  171.440788]  do_syscall_64+0x6e/0x2c0
[  171.440802]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
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/20200309110934.868-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.h

@@ -396,7 +396,9 @@ static inline bool i915_seqno_passed(u32 seq1, u32 seq2)
 
 static inline u32 __hwsp_seqno(const struct i915_request *rq)
 {
-	return READ_ONCE(*rq->hwsp_seqno);
+	const u32 *hwsp = READ_ONCE(rq->hwsp_seqno);
+
+	return READ_ONCE(*hwsp);
 }
 
 /**
@@ -510,7 +512,8 @@ static inline bool i915_request_completed(const struct i915_request *rq)
 
 static inline void i915_request_mark_complete(struct i915_request *rq)
 {
-	rq->hwsp_seqno = (u32 *)&rq->fence.seqno; /* decouple from HWSP */
+	WRITE_ONCE(rq->hwsp_seqno, /* decouple from HWSP */
+		   (u32 *)&rq->fence.seqno);
 }
 
 static inline bool i915_request_has_waitboost(const struct i915_request *rq)