2008-11-25 16:38:55

by Aneesh Kumar K.V

[permalink] [raw]
Subject: [PATCH -V3] 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 ..

FILENAME: fix-race-between-read_block_bitmap-and-mark_diskspace_used

Signed-off-by: Aneesh Kumar K.V <[email protected]>
fs/ext4/mballoc.c | 15 ++++++++++-----
1 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 7b5e968..aa03716 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1073,7 +1073,10 @@ static void mb_clear_bits(spinlock_t *lock, void *bm, int cur, int len)
cur += 32;
- mb_clear_bit_atomic(lock, cur, bm);
+ if (lock)
+ mb_clear_bit_atomic(lock, cur, bm);
+ else
+ mb_clear_bit(cur, bm);
@@ -1091,7 +1094,10 @@ static void mb_set_bits(spinlock_t *lock, void *bm, int cur, int len)
cur += 32;
- mb_set_bit_atomic(lock, cur, bm);
+ if (lock)
+ mb_set_bit_atomic(lock, cur, bm);
+ else
+ mb_set_bit(cur, bm);
@@ -2991,10 +2997,9 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
- 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);