Re: [RFC PATCH 0/2] Fix up the NFS mmap code

From: Al Viro
Date: Fri Jan 08 2010 - 21:11:38 EST

On Fri, Jan 08, 2010 at 05:57:27PM -0800, Linus Torvalds wrote:
> On Sat, 9 Jan 2010, Al Viro wrote:
> >
> > readdir() is certainly a red herring.
> That's the one that lockdep reports, though. I still don't see why. Afaik,
> the only place where NFS gets an inode is nfs_fhget(), and that seems to
> do things correctly.

Well, sure - it steps on i_mutex-before-mmmap_sem first from ls somewhere and
records the ordering for posterity. Then NFS steps into mmap() (on a
different inode) and gets conflicting ordering.

It would be a false positive if rules for NFS *really* had been different
and it could safely grab i_mutex on NFS inodes inside mmap_sem. It can't.
The rules really are the same. And readdir is just the earliest case of
kernel stepping on mmap_sem while holding *some* i_mutex. write() is
another and there i_mutex can very well be the same as in case of mmap().

lockdep doesn't make a distinction (and really, how many paths reinforcing
the normal lock ordering would you record?), but if we'd given i_mutex of
NFS regular files a class of its own, we'd see a warning with nfs write
instead of readdir...
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