Re: [PATCH -V2 3/5] ext4: Fix the race between read_block_bitmap and mark_diskspace_used

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Fri, Nov 21, 2008 at 11:22:04AM -0600, Eric Sandeen wrote:
> Aneesh Kumar K.V wrote:
> > We need to make sure we update the block bitmap and clear
> > EXT4_BG_BLOCK_UNINIT flag with sb_bgl_lock held. We look
> > at EXT4_BG_BLOCK_UNINIT and reinit the block bitmap each
> > time in ext4_read_block_bitmap (introduced by
> > c806e68f5647109350ec546fee5b526962970fd2 )
> 
> Can you add details about the failure mode(s) of this race, so people
> (i.e. me) have an idea which bugs they've seen that it might address?
> 

ext4_read_block_bitmap does

	spin_lock(sb_bgl_lock(EXT4_SB(sb), block_group));
	if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
		ext4_init_block_bitmap(sb, bh, block_group, desc);

the above ext4_init_block_bitmap actually zero out the block bitmap.

Now on the block allocation side we do

	mb_set_bits(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group), bitmap_bh->b_data,
				ac->ac_b_ex.fe_start, ac->ac_b_ex.fe_len);

	spin_lock(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group));
	if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
		gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);

ie on allocation we update the bitmap then we take the sb_bgl_lock
and clear the EXT4_BG_BLOCK_UNINIT flag. What can happen is a
parallel ext4_read_block_bitmap can zero out the bitmap in between
the above mb_set_bits and spin_lock(sb_bg_lock..)

Result of this race is
a) blocks getting allocated multiple times
b) File corruption because two files have same blocks allocated
c) mb_free_blocks called multiple times on the same block

....

Same is true with inode bitmap also.

-aneesh