Commit 08db141b authored Jun 22, 2017 by Jeff Mahoney Committed by Jan Kara Jun 23, 2017

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: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

parent 54930dfe

fs/reiserfs/bitmap.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -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)

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