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reiserfs: fix race in prealloc discard

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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@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Jeff Mahoney 2017-06-22 16:47:34 -04:00 committed by Jan Kara
parent 54930dfeb4
commit 08db141b53
1 changed files with 10 additions and 2 deletions
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12
fs/reiserfs/bitmap.c
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@ -513,9 +513,17 @@ static void __discard_prealloc(struct reiserfs_transaction_handle *th,
"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)