From 487caeef9fc08c0565e082c40a8aaf58dad92bbb Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Mon, 17 Aug 2009 22:17:20 -0400 Subject: ext4: Fix possible deadlock between ext4_truncate() and ext4_get_blocks() During truncate we are sometimes forced to start a new transaction as the amount of blocks to be journaled is both quite large and hard to predict. So far we restarted a transaction while holding i_data_sem and that violates lock ordering because i_data_sem ranks below a transaction start (and it can lead to a real deadlock with ext4_get_blocks() mapping blocks in some page while having a transaction open). We fix the problem by dropping the i_data_sem before restarting the transaction and acquire it afterwards. It's slightly subtle that this works: 1) By the time ext4_truncate() is called, all the page cache for the truncated part of the file is dropped so get_block() should not be called on it (we only have to invalidate extent cache after we reacquire i_data_sem because some extent from not-truncated part could extend also into the part we are going to truncate). 2) Writes, migrate or defrag hold i_mutex so they are stopped for all the time of the truncate. This bug has been found and analyzed by Theodore Tso . Signed-off-by: Jan Kara Signed-off-by: "Theodore Ts'o" --- fs/ext4/inode.c | 23 +++++++++++++++++++---- 1 file changed, 19 insertions(+), 4 deletions(-) (limited to 'fs/ext4/inode.c') diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 9a4c929b16dc..d61fb523308f 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -192,11 +192,24 @@ static int try_to_extend_transaction(handle_t *handle, struct inode *inode) * so before we call here everything must be consistently dirtied against * this transaction. */ -static int ext4_journal_test_restart(handle_t *handle, struct inode *inode) + int ext4_truncate_restart_trans(handle_t *handle, struct inode *inode, + int nblocks) { + int ret; + + /* + * Drop i_data_sem to avoid deadlock with ext4_get_blocks At this + * moment, get_block can be called only for blocks inside i_size since + * page cache has been already dropped and writes are blocked by + * i_mutex. So we can safely drop the i_data_sem here. + */ BUG_ON(EXT4_JOURNAL(inode) == NULL); jbd_debug(2, "restarting handle %p\n", handle); - return ext4_journal_restart(handle, blocks_for_truncate(inode)); + up_write(&EXT4_I(inode)->i_data_sem); + ret = ext4_journal_restart(handle, blocks_for_truncate(inode)); + down_write(&EXT4_I(inode)->i_data_sem); + + return ret; } /* @@ -3658,7 +3671,8 @@ static void ext4_clear_blocks(handle_t *handle, struct inode *inode, ext4_handle_dirty_metadata(handle, inode, bh); } ext4_mark_inode_dirty(handle, inode); - ext4_journal_test_restart(handle, inode); + ext4_truncate_restart_trans(handle, inode, + blocks_for_truncate(inode)); if (bh) { BUFFER_TRACE(bh, "retaking write access"); ext4_journal_get_write_access(handle, bh); @@ -3869,7 +3883,8 @@ static void ext4_free_branches(handle_t *handle, struct inode *inode, return; if (try_to_extend_transaction(handle, inode)) { ext4_mark_inode_dirty(handle, inode); - ext4_journal_test_restart(handle, inode); + ext4_truncate_restart_trans(handle, inode, + blocks_for_truncate(inode)); } ext4_free_blocks(handle, inode, nr, 1, 1); -- cgit v1.2.3