Mike Galbraith [off-list ref] :
[...]

Hrm, relatively new + tasklet woes rings a bell.  Ah, that..

<quote>
What's worse is that at the point where this code was written it was
already well known that tasklets are a steaming pile of crap and
should die.
</quote>

Source thereof https://lwn.net/Articles/588457/

tasklets are ingrained in the dmaengine API (see Documentation/dmaengine/client.txt
and drivers/dma/virt-dma.h::vchan_cookie_complete).

Moving everything to irq context or handling his own sub-{jiffy/ms} timer
while losing async dma doesn't exactly smell like roses either. :o(

-- 
Ueimor