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    SubjectRe: [git pull] vfs and fs fixes
    On Fri, Apr 20, 2012 at 01:15:17PM +0200, Jan Kara wrote:
    > On Wed 18-04-12 00:44:24, 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", since there we don't get to change the locking order - the first
    > > ->i_mutex is taken too far outside.
    > Hum, I think I could just do away with quota file i_mutex being special.
    > It's used for two purposes:
    > 1) When quota is being turned on/off, we want to set/clear inode immutable
    > flag, truncate page cache, etc. But we should be able push this locking
    > outside of quota locks.
    > 2) Inside filesystems when quota file is written to. Quota writes are
    > serialized by quota code anyway and noone else has any bussiness with quota
    > files (they are marked as immutable to avoid mistakes) so there i_mutex is
    > not really needed.

    Grepping for I_MUTEX_QUOTA shows hits in ext4, reiserfs, and gfs2. The
    former two are in code called from the quota code (through the
    ->quota_write method). But the gfs2 code appears to be called directly
    from gfs2's write code.


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