Quoting Chris Wilson (2019-11-23 17:49:46)
> Since we want to do a lockless read of the current active request, and
> that request is written to by process_csb also without serialisation, we
> need to instruct gcc to take care in reading the pointer itself.
>
> Otherwise, we have observed execlists_active() to report 0x40.
>
> [ 2400.760381] igt/para-4098 1..s. 2376479300us : process_csb: rcs0 cs-irq head=3, tail=4
> [ 2400.760826] igt/para-4098 1..s. 2376479303us : process_csb: rcs0 csb[4]: status=0x00000001:0x00000000
> [ 2400.761271] igt/para-4098 1..s. 2376479306us : trace_ports: rcs0: promote { b9c59:2622, b9c55:2624 }
> [ 2400.761726] igt/para-4097 0d... 2376479311us : __i915_schedule: rcs0: -2147483648->3, inflight:0000000000000040, rq:ffff888208c1e940
>
> which is impossible!
>
Fixes: df403069029d ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
-Chris
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