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 <tytso@mit.edu>.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 13db434..8c20caf 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -93,7 +93,9 @@
ix->ei_leaf_hi = cpu_to_le16((unsigned long) ((pb >> 31) >> 1) & 0xffff);
}
-static int ext4_ext_journal_restart(handle_t *handle, int needed)
+static int ext4_ext_truncate_extend_restart(handle_t *handle,
+ struct inode *inode,
+ int needed)
{
int err;
@@ -104,7 +106,14 @@
err = ext4_journal_extend(handle, needed);
if (err <= 0)
return err;
- return ext4_journal_restart(handle, needed);
+ err = ext4_truncate_restart_trans(handle, inode, needed);
+ /*
+ * We have dropped i_data_sem so someone might have cached again
+ * an extent we are going to truncate.
+ */
+ ext4_ext_invalidate_cache(inode);
+
+ return err;
}
/*
@@ -2150,7 +2159,7 @@
}
credits += 2 * EXT4_QUOTA_TRANS_BLOCKS(inode->i_sb);
- err = ext4_ext_journal_restart(handle, credits);
+ err = ext4_ext_truncate_extend_restart(handle, inode, credits);
if (err)
goto out;