linux-kernel - Re: [git pull] vfs and fs fixes

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Message-ID: <20120418215238.GA11959@fieldses.org>
Date:	Wed, 18 Apr 2012 17:52:38 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [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?

--b.

> , 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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