Commit 08db141b authored by Jeff Mahoney's avatar Jeff Mahoney Committed by Jan Kara

reiserfs: fix race in prealloc discard

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: 's avatarJeff Mahoney <>
Signed-off-by: 's avatarJan Kara <>
parent 54930dfe
......@@ -513,9 +513,17 @@ static void __discard_prealloc(struct reiserfs_transaction_handle *th,
"inode has negative prealloc blocks count.");
while (ei->i_prealloc_count > 0) {
reiserfs_free_prealloc_block(th, inode, 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++;
reiserfs_free_prealloc_block(th, inode, block_to_free);
dirty = 1;
if (dirty)
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