The main loop in __discard_prealloc is protected by the reiserfs write lock
which is dropped across schedules like the BKL it replaced. The problem is
that it checks the value, calls a routine that schedules, and then adjusts
the state. As a result, two threads that are calling
reiserfs_prealloc_discard at the same time can race when one calls
reiserfs_free_prealloc_block, the lock is dropped, and the other calls
reiserfs_free_prealloc_block with the same block number. In the right
circumstances, it can cause the prealloc count to go negative.
Signed-off-by: Jeff Mahoney <jeffm@xxxxxxxx>
---
bitmap.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
--- a/fs/reiserfs/bitmap.c
+++ b/fs/reiserfs/bitmap.c
@@ -479,9 +479,17 @@ static void __discard_prealloc(struct re
"inode has negative prealloc blocks count.");
#endif
while (ei->i_prealloc_count > 0) {
- reiserfs_free_prealloc_block(th, inode, ei->i_prealloc_block);
- ei->i_prealloc_block++;
+ b_blocknr_t block_to_free;
+
+ /*
+ * reiserfs_free_prealloc_block can drop the write lock,
+ * which could allow another caller to free the same block.
+ * We can protect against it by modifying the prealloc
+ * state before calling it.
+ */
+ block_to_free = ei->i_prealloc_block++;
ei->i_prealloc_count--;
+ reiserfs_free_prealloc_block(th, inode, block_to_free);
dirty = 1;
}
if (dirty)
--
Jeff Mahoney
SUSE Labs
--
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