Quoting Tvrtko Ursulin (2018-02-19 09:25:00)
> On 16/02/2018 10:13, Chris Wilson wrote:
> > Afaict, it's complaining that because the writer is special the reader
> > needs irq protection. Which I don't understand, as the point is that the
> > writer can run concurrently to the readers (the readers have to
> > restart). What am I missing?
> As discussed on IRC, issue is courtesy of perf read callback running in 
> hardirq context, that the reader can interrupt the writer and so 
> incorrectly declare a stable sequence number while reading the mixed up 
> version of the underlying data.
> Writer:
>    seqno++
>    modify some fields
>    <Reader IRQ...
>      read seqno
>      read fields
>      re-read seqno -> OK
>    >..Reader IRQ
>    modify other fields
>    seqno++
> So we need to go back to the first, irqsave version. Do you want to 
> pursue this change right now or leave it for some future work?

It's a small optimisation (hopefully ;) so pursue at leisure or in the
context of wider work.
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