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PCI hot-plug: use dedicated taskqueue for device attach / detach
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Authored by avg on May 6 2021, 5:55 AM.

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Summary

Attaching and detaching devices can be heavy-weight and detaching can
sleep waiting for events. For that reason using the system-wide
single-threaded taskqueue_thread is not really appropriate.
There is even a possibility for a deadlock if taskqueue_thread is used
for detaching.

In fact, there is an easy to reproduce deadlock involving nvme, pass
and a sudden removal of an NVMe device.
A pass peripheral would not release a reference on an nvme sim until
pass_shutdown_kqueue() is executed via taskqueue_thread. But the
taskqueue's thread is blocked in nvme_detach() -> ... -> cam_sim_free()
because of the outstanding reference.

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