From: Alex Zhuravlev <Alex.Zhuravlev@Sun.COM>
Subject: Re: [PATCH -V2 3/5] ext4: Fix the race between read_block_bitmap and
 mark_diskspace_used
Date: Mon, 24 Nov 2008 21:17:53 +0300
Message-ID: <492AEFD1.4060701@sun.com>
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Cc: Theodore Tso <tytso@MIT.EDU>, cmm@us.ibm.com, sandeen@redhat.com,
	linux-ext4@vger.kernel.org
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
In-reply-to: <20081124181252.GE8462@skywalker>
Sender: linux-ext4-owner@vger.kernel.org

Aneesh Kumar K.V wrote:
> With commit c806e68f we do a init_bitmap every time we do a
> read_block_bitmap.

can you explain why do we need to init it every time?

thanks, Alex

> 
> To quote the update commit message that i have
> 
>     ext4: Fix race between read_block_bitmap() and mark_diskspace_used()
>     
>     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 commit c806e68f), and this can
>     race with block allocations in ext4_mb_mark_diskspace_used().
>     
>     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);
>     
>     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..)
>     
>     The race results in below user visible errors
>     EXT4-fs error (device sdb1): ext4_mb_release_inode_pa: free 100, pa_free 105
>     EXT4-fs error (device sdb1): mb_free_blocks: double-free of inode 0's block 50(bit 100 in group 0)
> 
> -aneesh