[RFC] Understanding the locking of struct buf

[Alexander Lochmann]

Hi folks!

According to the definition [1], any access to a struct buf is protected
by a lock:
1) Are there exceptions to those locking rules for reading and/or
writing a member?
E.g. any read is permitted without a lock being held.
Can b_bcount, for example, be read without a lock?

2) In vfs_bio.c, for example, the BUF_KERNPROC macro is used to move
ownership of a buf.b_lock to the kernel.
In _lockmgr_disown() [2] WITNESS_UNLOCK() is called to emulate an
unlock. So, shouldn't be there a WITNESS_LOCK() for that lock?
Otherwise, the Witness system might complain about the upcoming unlock.

3) Is the function brelse()/bqrelse() considered to be a destruction
function?
Hence, no lock (buf.b_lock) is needed within this function to access a
struct buf.

Thanks!

Regards,
Alex

[1] https://github.com/freebsd/freebsd-src/blob/main/sys/sys/buf.h#L99
[2]
https://github.com/freebsd/freebsd-src/blob/main/sys/kern/kern_lock.c#L1616
[3] https://github.com/freebsd/freebsd-src/blob/main/sys/sys/buf.h#L387

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