Re: RT and XFS
From: Daniel Walker (dwalker_at_mvista.com)
Date: 07/13/05
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To: Nathan Scott <nathans@sgi.com> Date: Tue, 12 Jul 2005 17:41:43 -0700
On Wed, 2005-07-13 at 10:25 +1000, Nathan Scott wrote:
> On Tue, Jul 12, 2005 at 04:01:32PM -0700, Daniel Walker wrote:
> >
> > Is there something so odd about the XFS locking, that it can't use the
> > rt_lock ?
>
> Not that I know of - XFS does use the downgrade_write interface,
> whose use isn't overly common in the rest of the kernel... maybe
> that has caused some confusion, dunno.
Current RT doesn't implement downgrade_write() , but it's trivial to add
it.
> > --- linux.orig/fs/xfs/linux-2.6/mrlock.h
> > +++ linux/fs/xfs/linux-2.6/mrlock.h
> > @@ -37,12 +37,12 @@
> > enum { MR_NONE, MR_ACCESS, MR_UPDATE };
> >
> > typedef struct {
> > - struct rw_semaphore mr_lock;
> > - int mr_writer;
> > + struct compat_rw_semaphore mr_lock;
> > + int mr_writer;
> > } mrlock_t;
>
> The XFS code is also written such that it just releases a mrlock
> without tracking whether it had it for access/update in the end
> (end lock state is not necessarily how it started out, since it
> may have downgraded the lock at some point, or it may not have).
> Its a non-trivial change to track that state within XFS itself,
> so the above mr_writer field in XFS's mrlock wrapper tracks that
> state alongside the rw_semaphore. It would prefer to be getting
> that out of the rw_semaphore itself, alot, but there's not any
> mechanism for doing so (its not a particularly nice API change
> either, really, for the generic locking code). I guess that may
> have been another reason for the above change in the RT patch, I
> don't know all the details there.
So it calls up_read if it has a read lock ? Or up_write if it has a
write lock? I suppose it would be broken if it didn't though.
Daniel
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