Alexander Viro writes:
>
>
>
> On Mon, 1 May 2000, Richard Gooch wrote:
>
> > Eric W. Biederman writes:
> > > Richard Gooch <[EMAIL PROTECTED]> writes:
> > >
> > > > Hi, Al. You've previously stated that you consider the multiple
> > > > mount feature of devfs broken. I agree that there are some races in
> > > > there. However, I'm not clear on whether you're saying that the entire
> > > > concept is broken, or that it can be fixed with appropriate loffcking.
> > > > I've asked this before, but haven't had a response.
> > >
> > > Last I saw it was his complaint that you varied what you
> > > showed at different mount points, and that doing that all in
> > > one dcache tree was fundamentally broken.
> >
> > But it's not one dcache tree: there is a separate dcache tree for each
> > mount of devfs. So I don't understand that complaint.
>
> There is a lot of places where we do serialization using semaphores in
> struct inode. You don't duplicate it all. Think what happens if two
> instances operate on the same directory. You've got no locking here.
Correct, I have no locking now. But adding locking is on my list of
things to do. So, for example, I could put a semaphore in my "struct
directory_type" (amongst other places). That would serialise changes.
I get the impression you think no amount of locking added to devfs
will solve the races, but I don't see why that would be the case.
Or are you just saying that it would be a lot of work to get all the
locking correct? If that's what bothers you, don't worry. That's my
job ;-)
> > > > If you feel that it's fundamentally impossible to mount a FS multiple
> > > > times, please explain your reasoning.
> > >
> > > At this point it would make sense to just use the generic multiple
> > > mount features in the VFS that Alexander has been putting in.
> >
> > The generic multi-mount patch is good, but it doesn't solve the
> > particular problem of mounting with selective exposure.
>
> There is one case when you can safely have multiple trees - when
> directories in each tree are read-only. If you union-mount such
> trees you get some selective exposure.
Yes, I've been thinking about this approach. I'll like to table it for
now and sort the locking issue out first.
BTW: when are we likely to have union mounts available?
Regards,
Richard....
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