virtio_p9fs: Fix kernel panic on module unload

[arichardson]

Summary

The virtio_p9fs module event handler can be invoked multiple times.
Previously, this caused p9_init_zones() and p9_register_trans() to be
executed multiple times, leaking UMA zones and corrupting the transport
list. During module unload, p9_destroy_zones() was also called multiple
times on the same zone pointers, triggering a duplicate free kernel panic
in uma_zdestroy().

This patch introduces a static reference counter in vt9p_modevent() to
ensure the zones and transports are only initialized and destroyed exactly
once, aligning with the approach used by other virtio drivers like vtnet.

[zlei]

sys/dev/virtio/p9fs/virtio_p9fs.c

479

This makes me nervous about the whole lifecycle of a module. I noticed that the module event will be only invoked exactly once ( if successfully ) while I was hacking the SYSINITs .

The virtio_p9fs module event handler can be invoked multiple times.

How does it possible to happen ?

[zlei]

sys/dev/virtio/p9fs/virtio_p9fs.c

493

Ahh, I see what happened.

VIRTIO_DRIVER_MODULE expands to two different DRIVER_MODULE and both of them share the same modevent handler vt9p_modevent. So on module load, the event handler vt9p_modevent will be invoked twice.