From owner-freebsd-fs  Sat Jun 27 00:34:48 1998
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Date: Sat, 27 Jun 1998 17:34:37 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199806270734.RAA02480@godzilla.zeta.org.au>
To: fs@FreeBSD.ORG, phk@FreeBSD.ORG
Subject: Re: specfs oddity...
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>Does anybody know why spec_open for VCHR devices does a VOP_UNLOCK() /
>vn_lock(vp, LK_EXCLUSIVE) around the devicedriver open routine but
>doesn't do it for VBLK devices ?

I think the vnode is left locked for bdev opens because unlocking
would allow the !vfs_mountedon(vp) condition to change underneath.

The vnode obviously has to be kept locked for bdev reads and writes
because the inner i/o function (bread/bwrite) depends on the vnode.
Unlocking/not locking is apparently safe for the other specfs
operations (ioctls and closes are not locked to begin with).

Bruce

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