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    SubjectRe: [git pull] vfs and fs fixes
    On Wed, Apr 18, 2012 at 12:44:24AM +0100, Al Viro wrote:
    > On Tue, Apr 17, 2012 at 03:08:26PM -0700, Linus Torvalds wrote:
    > > > Or I could increment that counter for all the conflicting operations and
    > > > rely on it instead of the i_mutex. ?I was trying to avoid adding
    > > > something like that (an inc, a dec, another error path) to every
    > > > operation. ?And hoping to avoid adding another field to struct inode.
    > > > Oh well.
    > >
    > > We could just say that we can do a double inode lock, but then
    > > standardize on the order. And the only sane order is comparing inode
    > > pointers, not inode numbers like ext4 apparently does.
    > >
    > > With a standard order, I don't think it would be at all wrong to just
    > > take the inode lock on rename.
    > In principle, yes, but have you tried to grep for i_mutex? Note that
    > we have *another* place where multiple ->i_mutex might be held on
    > non-directories (and unless I'm missing something, ext4 move_extent.c
    > stuff doesn't play well with it): quota writes. Which can, AFAICS,
    > happen while write(2) is holding ->i_mutex on a regular file. So
    > it's not _that_ easy - we want something like "and quota file is goes
    > last"

    So the idea would be to always take the i_mutex on non-quota files
    before taking it on quota files?

    I tried pulling the ext4 thing into fs/inode.c, modifying the order to
    do that, and then doing the rename change on top of that.

    One thing I don't understand is how that interacts with
    quota_on/quota_off. How do we decide the right lock ordering if files
    can go back and forth between being quota files?


    > , since there we don't get to change the locking order - the first
    > ->i_mutex is taken too far outside.
    > I really don't like how messy i_mutex had become these days. Right now
    > I'm staring at 700-odd lines all over the place where it's taken/released
    > and it's a wastebucket lock - used to protect random bits and scraps, with a
    > lot of filesystems, etc. using it for purposes of their own ;-/

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