svn commit: r208003 - in head/sys: kern sys

[Kostik Belousov]

On Mon, May 17, 2010 at 08:33:52AM -0700, Matthew Fleming wrote:
> > From: Kostik Belousov [mailto:kostikbel at gmail.com]
> > On Wed, May 12, 2010 at 09:24:46PM +0000, Zachary Loafman wrote:
> > > Author: zml
> > > Date: Wed May 12 21:24:46 2010
> > > New Revision: 208003
> > > URL: http://svn.freebsd.org/changeset/base/208003
> > > 
> > > Log:
> > >   Add VOP_ADVLOCKPURGE so that the file system is called when purging
> > >   locks (in the case where the VFS impl isn't using lf_*)
> > >   
> > >   Submitted by:       Matthew Fleming <matthew.fleming at isilon.com>
> > >   Reviewed by:        zml, dfr
> > 
> > After looking at what happen to nullfs, see r208003, I wonder why
> > the vop is needed. It is called after VOP_RECLAIM is called by vgonel(),
> > after fs-specific data are destroyed. So, on the one hand, vop can only
> > operate on struct vnode proper, on the other hand, the actions performed
> > by vop_advlockpurge implementation can be done by vop_reclaim as well.
> > 
> > Could you, please, give some details on the supposed use of the vop ?
> 
> >From a design perspective, it makes little sense to allow overriding
> the advlock operation, but not the purge. A specific example is if an
> implementation does not use struct lockf to implement advlock, then
> the hack you mention of purging in VOP_RECLAIM is needed.
>
> After looking over the history of the changes, I believe it would be
> sufficient to have the lock purge done before the reclaim. The vnode
> is locked exclusively for both operations, so I don't believe there
> will be any timing windows. But I am still not 100% sure when the file
> lock is used versus the interlock for serializing access to various
> fields.
>
> The advlock VOPs are analogous to the regular VOP_LOCK in that we
> expect an implementation may override the VOP and also the data
> structures used, e.g. to not use the vnode's v_lock field for the
> VOP_LOCK. Thus any code which refers to v_lockf should be wrapped in a
> VOP to allow correct overriding.

Essentially, my argument is that whatever you do in VOP_ADVLOCKPURGE,
can be and should be done in VOP_RECLAIM. This would also cover the v_data
issue.
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