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

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Date:	Thu, 3 Apr 2008 15:07:00 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	"Josef 'Jeff' Sipek" <jeffpc@...efsipek.net>
Cc:	Erez Zadok <ezk@...sunysb.edu>, 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 

In message <20080403182001.GB30189@...efsipek.net>, "Josef 'Jeff' Sipek" writes:
> On Wed, Apr 02, 2008 at 09:49:11PM -0400, Erez Zadok wrote:
> ...
> > +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
> > +	spin_lock(&dst->i_lock);
> > +#endif
> 
> I think you need to check CONFIG_PREEMPT as well.
> 
> Josef 'Jeff' Sipek.

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.

/*
 * NOTE: unlike i_size_read(), i_size_write() does need locking around it
 * (normally i_mutex), otherwise on 32bit/SMP an update of i_size_seqcount
 * can be lost, resulting in subsequent i_size_read() calls spinning forever.
 */
static inline void i_size_write(struct inode *inode, loff_t i_size)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	write_seqcount_begin(&inode->i_size_seqcount);
	inode->i_size = i_size;
	write_seqcount_end(&inode->i_size_seqcount);
#elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPT)
	preempt_disable();
	inode->i_size = i_size;
	preempt_enable();
#else
	inode->i_size = i_size;
#endif
}


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
- 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)

Erez.
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