Understanding locking for buf

Hi Konstantin!

I'm sorry. I'm still struggling to understand locking of struct buf.
As far as I know, a struct buf is (mostly) protected by b_lock.
You already explained the concept of LK_KERNPROC.

I, however, have an instance of struct buf. That particular one is 
mostly accessed from just one context (aka thread) -- assuming 
synchronous IO.
The other, unprotected accesses come from just one other context. Those 
accesses originate from 'g_vfs_done' [1].
Why don't you release b_lock when buf goes further down the io stack, 
and acquire it again in g_vfs_done?
This way the context of g_vfs_done would own the b_lock.

Viewing it from a different angle: Are accesses in g_vfs_done safe 
because the buf instance is already locked from a global perspective?
Hence, other code paths would block on BUF_LOCK().

Regards,
Alex

[1] 
https://iris.cs.tu-dortmund.de/freebsd-lockdoc/latest/source/sys/geom/geom_vfs.c#L96
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