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SubjectRe: RT and XFS
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.

> --- 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.

cheers.

--
Nathan
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