linux-kernel - Re: [2.6.26 PATCH, RESEND]: fs_stack/eCryptfs: fsstack_copy

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Date:	Thu, 1 May 2008 19:44:18 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Erez Zadok <ezk@...sunysb.edu>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	hch@...radead.org, mhalcrow@...ibm.com, hugh@...itas.com
Subject: Re: [2.6.26 PATCH, RESEND]: fs_stack/eCryptfs: fsstack_copy_* updates 

In message <20080501122136.bc2215c9.akpm@...ux-foundation.org>, Andrew Morton writes:
> On Thu, 1 May 2008 13:18:09 -0400
> Erez Zadok <ezk@...sunysb.edu> wrote:
> 
> > In message <20080430101704.9cbd6384.akpm@...ux-foundation.org>, Andrew Morton writes:
> > [...[
> > > Can we avoid having to think?
> > > 
> > > void fsstack_copy_inode_size(struct inode *dst, const struct inode *src)
> > > {
> > > 	blkcnt_t i_blocks;
> > > 	loff_t i_size;
> > > 
> > > 	i_size = i_size_read(src);
> > > 	spin_lock_32bit(&src->i_lock);
> > > 	i_blocks = src->i_blocks;
> > > 	spin_unlock_32bit(&src->i_lock);
> > > 
> > > 	i_size_write(dst, i_size);
> > > 	spin_lock_32bit(&dst->i_lock)
> > > 	dst->i_blocks = i_blocks;
> > > 	spin_unlock_32bit(&dst->i_lock)
> > > }
> > 
> > I can't find spin_[um]lock_32bit anywhere (checkd latest mmotm and linus's
> > tree).  I therefore assume this was just your way of saying I should:
> > 
> > #if BITS_PER_LONG == 32
> > 	spin_unlock(&dst->i_lock);
> > #endif
> 
> Nope, it was my way of suggesting that you implement it ;)
> include/linux/spinlock.h would be a good place.

Sure, I can implement it and submit a patch.  But I've got two questions
first:

1. i_size_read has a tri-state behaviour:

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	// play with seqcount
#elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPT)
	// wrap in preempt_disable/enable
#else
	// just return value
#endif

Shouldn't the same be done for i_blocks, with matching i_blocks_read(),
i_blocks_write(), and adding an inode->i_blocks_seqcount field?


2. I've rewritten your suggested code a bit to reduce stack use.  Modulo
   having 32-bit spin_lock/unlock variants, do you see any problem with this
   code below?  My testing of it so far on 32/64-bit SMP/UMP have all
   passed.

void fsstack_copy_inode_size(struct inode *dst, struct inode *src)
{
#if BITS_PER_LONG == 32
	blkcnt_t i_blocks;

	spin_lock(&src->i_lock);
	i_blocks = src->i_blocks;
	spin_unlock(&src->i_lock);
	spin_lock(&dst->i_lock);
	dst->i_blocks = i_blocks;
	spin_unlock(&dst->i_lock);
#else
	dst->i_blocks = src->i_blocks;
#endif
	i_size_write(dst, i_size_read(src));
}

Thanks,
Erez.
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