concourent access to a buffer

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
in quota2, the quota usage are read/updated directly in the disk buffer,
so that they are updated as part of the filesystem metadata.

When a quota entry is needed, the disk block containing it is retrieved
with bread() (which marks the disk buffer B_BUSY), the entry is
used/updated and it is then unbusied with bwrite() or brelse().

Is B_BUSY enough to guarantee exclusive access to the data, or is some
extra locking needed ?
Is there a way to share a buffer read-only between different threads ?

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--