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    SubjectRe: missing mnt_drop_write() on open error
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    On Wed, 2007-09-26 at 10:38 +0200, Miklos Szeredi wrote: 
    > > On Wed, 2007-09-26 at 01:14 +0200, Miklos Szeredi wrote:
    > > > I get this at umount, if there was a failed open():
    > > >
    > > > WARNING: at fs/namespace.c:586 __mntput()
    > > >
    > > > I think the problem is that may_open() calls mnt_want_write(), but if
    > > > open doesn't succeed, mnt_drop_write() will not be called.
    > >
    > > Does this help?
    >
    > It didn't fix it for me, but the patch looks OK.
    >
    > In __dentry_open() there's still a few places where fput() won't be
    > called, notably when ->open fails, which is what I'm triggering I
    > think.
    >
    > Also even more horrible things can happen because of the
    > nd->intent.open.file thing. For example if the lookup routine calls
    > lookup_instantiate_filp(), and after this, but before may_open() some
    > error happens, then release_open_intent() will call fput() on the
    > file, which will cause mnt_drop_write() to be called, even though a
    > matching mnt_want_write() hasn't yet been called. Ugly, eh?

    I used to have a patch that didn't completely trust that all files with
    FMODE_WRITE set to have taken a write on the mnt. I think I used a flag
    to indicate whether or not a particular file had a mnt_want_write() done
    on its behalf. It somewhat artificially keeps the mnt write count
    balanced, but I think it will let us detect when things like this go on.

    > > I'm also thinking that we should change the open_namei*
    > > functions to simply return 'struct file *'. Those are the only users
    > > other than NFS, and forcing the return of a file like that will force
    > > users to do the fput() on it if they don't want it any more. We'd just
    > > need to make sure no new may_open() users pop up. Any thoughts on that?
    >
    > Yeah, something needs to be done with open, because currently it's way
    > too convoluted.

    Sounds like Christoph has some ideas...

    -- Dave

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