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ext4: Fix race in ext4_inode_info.i_cached_extent

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If two CPU's simultaneously call ext4_ext_get_blocks() at the same
time, there is nothing protecting the i_cached_extent structure from
being used and updated at the same time.  This could potentially cause
the wrong location on disk to be read or written to, including
potentially causing the corruption of the block group descriptors
and/or inode table.

This bug has been in the ext4 code since almost the very beginning of
ext4's development.  Fortunately once the data is stored in the page
cache cache, ext4_get_blocks() doesn't need to be called, so trying to
replicate this problem to the point where we could identify its root
cause was *extremely* difficult.  Many thanks to Kevin Shanahan for
working over several months to be able to reproduce this easily so we
could finally nail down the cause of the corruption.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
This commit is contained in:
Theodore Ts'o 2009-05-15 09:07:28 -04:00
parent 2a8964d63d
commit 2ec0ae3ace
1 changed files with 12 additions and 5 deletions
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17
fs/ext4/extents.c
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@ -1841,11 +1841,13 @@ ext4_ext_put_in_cache(struct inode *inode, ext4_lblk_t block,
{
struct ext4_ext_cache *cex;
BUG_ON(len == 0);
spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
cex = &EXT4_I(inode)->i_cached_extent;
cex->ec_type = type;
cex->ec_block = block;
cex->ec_len = len;
cex->ec_start = start;
spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
}
/*
@ -1902,12 +1904,17 @@ ext4_ext_in_cache(struct inode *inode, ext4_lblk_t block,
struct ext4_extent *ex)
{
struct ext4_ext_cache *cex;
int ret = EXT4_EXT_CACHE_NO;
/*
* We borrow i_block_reservation_lock to protect i_cached_extent
*/
spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
cex = &EXT4_I(inode)->i_cached_extent;
/* has cache valid data? */
if (cex->ec_type == EXT4_EXT_CACHE_NO)
return EXT4_EXT_CACHE_NO;
goto errout;
BUG_ON(cex->ec_type != EXT4_EXT_CACHE_GAP &&
cex->ec_type != EXT4_EXT_CACHE_EXTENT);
@ -1918,11 +1925,11 @@ ext4_ext_in_cache(struct inode *inode, ext4_lblk_t block,
ext_debug("%u cached by %u:%u:%llu\n",
block,
cex->ec_block, cex->ec_len, cex->ec_start);
return cex->ec_type;
ret = cex->ec_type;
}
/* not in cache */
return EXT4_EXT_CACHE_NO;
errout:
spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
return ret;
}
/*