linux-kernel - Re: fs_stack/eCryptfs: remove 3rd arg of copy_attr_all, add locking to copy_inode

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Message-ID: <Pine.LNX.4.64.0804032014350.4616@blonde.site>
Date:	Thu, 3 Apr 2008 20:26:54 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Erez Zadok <ezk@...sunysb.edu>
cc:	"Josef 'Jeff' Sipek" <jeffpc@...efsipek.net>,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	hch@...radead.org
Subject: Re: fs_stack/eCryptfs: remove 3rd arg of copy_attr_all, add locking
 to copy_inode_size 

On Thu, 3 Apr 2008, Erez Zadok wrote:
> In message <20080403182001.GB30189@...efsipek.net>, "Josef 'Jeff' Sipek" writes:
> > I think you need to check CONFIG_PREEMPT as well.
> 
> I'm not sure if it's needed in case of CONFIG_PREEMPT.  Anyone?  The code
> for i_size_write (below), and the comment at the top of the function,
> suggest that the spinlock is needed only to prevent the lots seqcount.

Correct.

> BTW, some time ago I reviewed all callers of i_size_write.  I did so again
> just now, and the results were the same:
> 
> - a LOT of callers of i_size_write don't take any lock

They mostly know that i_mutex is already held (as i_size_write comment
mentions); but I believe that's up to the individual filesystem.

> - some take another spinlock in a different data structure
> - those that do take the spinlock, do so unconditionally
> - only unionfs and fs/stack.c wrap the spinlock in
> 
> 	#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)

I chose to follow the #ifdeffery of i_size_write(),
but you could do it unconditionally if you prefer:
just a little more overhead when it's not needed.

As I've said elsewhere, I don't think the result can be entirely
safe against concurrent changes in the lower filesystem, using
different locking; but I don't know how resilient unionfs is
expected to be against messing directly with lower at the same
time as upper level.

Hugh
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