I concur with this appraisal from Al Viro. Single threading the VFS is
going backwards -- not a good idea.
:-)
Jeff
Alexander Viro wrote:
>
> On Wed, 30 Aug 2000, Roman Zippel wrote:
>
> > > > hfs. For example reading from a file might require a read from a btree
> > > > file (extent file), with what another file write can be busy with (e.g.
> > > > reordering the btree nodes).
> > >
> > > And?
> >
> > The point is: the thing I like about Linux is its simple interfaces, it's
> > the basic idea of unix - keep it simple. That is true for most parts - the
> > basic idea is simple and the real complexity is hidden behind it. But
> > that's currently not true for vfs interface, a fs maintainer has to fight
> > right now with fscking complex vfs interface and with a possible fscking
>
> Yes? And it will become simpler if you will put each and every locking
> scheme into the API?
>
> Look: we have Hans with his trees-all-over-the-place + journal. He has a
> very legitimate need to protect the internal data structures of Reiserfs
> and do it without changing the VFS<->reiserfs interaction whenever he
> decides to change purely internal structures.
>
> We have ext2 with indirect blocks, inode bitmaps and block bitmaps, one
> per cylinder group + counters in each cylinder group. Should VFS know
> about the internal locking rules? Should it be aware of the fact that
> inodes foo and bar belong to the same cylinder group and if we remove them
> we will need to protect the bitmap for a while?
>
> We have FAT32 where we've got a nasty allocation data with rather
> interesting locking rules. Should it be protected by VFS? If it should -
> well, I have bad news for you: write() on a file will lock the whole
> filesystem until write() completes. Don't like it for every fs? Tough, it
> will mean that VFS will not protect the thing and fs will have to do it
> itself.
>
> We have AFFS with totally fscked directory structures. Do you propose to
> make unlink() block all directory operations on the whole fs? No? Too
> bad, because only AFFS knows enough to protect its data structures without
> _that_ locking. Sorry, the only rule that would not require the knowledge
> of layout and would be strong enough to protect is "no directory access
> while unlink() is in progress". Yup, on the whole fs. Hardly acceptable
> even for one filesystem, but try to impose that on everyone and see how
> long you will survive. JPEGs of the murder scene would be appreciated,
> BTW.
>
> We have HFS with the data structures of its own. You want locking in VFS
> that would protect the things VFS doesn't know about and has no business
> to meddle with? Fine, post the locking rules.
>
> It's insane - protection of purely internal data structures belongs to the
> module that knows about them. Generic stuff can, should be and _is_
> protected. Private one _can't_ be protected without either horribly
> crippled system (see above) or putting the knowledge of each data
> structure into the generic layer. And the latter will be on the author of
> filesystem anyway, because only he knows what rules he need.
>
> Please, propose your magical locking scheme that will protect everything
> on every fs. And let maintainers of filesystems tell you whether it is
> sufficient. Then check what's left after that locking - e.g. can two
> processes access the same fs at the same time or not?
>
> If you are complaining about the fact that maintaining complex data
> structures in multithreaded program (which kernel is) may be, well,
> complex - welcome to reality. It had been that way since the very
> beginning on _all_ MT projects, Linux included. You have complex private
> data - you may be in for pain protecting yourself from races. Protection
> of the public structures is there, so life became easier than it used to
> be back in 2.0/2.1/2.2 days.
>
> Making VFS single-threaded will not fly. If you can show simpler MT one -
> do it and a lot of people will be extremely grateful. 4.4BSD and SunOS
> ones are more complex and make the life harder for filesystem writers.
> Check yourself. OSF/1 is _much_ more complex. Hell knows what NT has, but
> filesystem glue there looks absolutely horrible - compared to them we are
> angels in that respect. v7 was simpler, sure enough. Without mmap(),
> rename() and truncate() _and_ with only one fs type - why not? Too bad
> that it was racey as hell... Plan 9 is nice and easy. Without mmap(),
> without link(), without truncate(), without cross-directory rename() and
> without support of crazy abortions from hell a-la AFFS. 2.0 and 2.2 are
> _way_ more complex, just compare filesystem code size in 2.4 with them and
> you will see. And yes, races in question are not new. I can reproduce them
> on 2.0.9 box. Single-processor one - nothing fancy and SMP-related.
>
> If you have a way to simplify VFS and/or filesystems - by all means,
> post it on fsdevel/l-k. Just tell what locking warranties you provide.
> Current ones are documented in the tree, so it will be very easy to
> compare. I'm not saying that they are ideal (check the documentation in
> question - I'm saying the opposite in quite a few cases). They _can_ be
> made better. But if you are saying that you know how to protect purely
> internal data structures without losing MT VFS - sorry, I will believe it
> when I see it. Go ahead, prove me wrong.
>
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