Re: [git pull] vfs and fs fixes

["J. Bruce Fields" <bfields@fieldses.org>]

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.

--b.