Hi, > Here is a question to be confirmed. > > In ext3_ioctl() with "cmd == EXT3_IOC_SETFLAGS", we firstly lock > "inode->i_mutex", start a handle with 1 journal-block by calling > ext3_journal_start(). In ext3_new_blocks(), say QUOTA was enabled with vfsv0 > format, we will call the function "DQUOT_ALLOC_BLOCK()". The handle in > ext3_new_blocks() was started by high-level functions, and > DQUOT_ALLOC_BLOCK() will finally calles ext3_quota_write() in which it try > to lock the "i_mutex" of the inode of a quota-file. > > At it happens, when we want to modify the inodes of quota-files via > ext3_ioctl(cmd = EXT3_IOC_SETFLAGS) (say process-A), another guy try to > execute ext3_quota_write() by calling DQUOT_ALLOC_BLOCK() (say process-B). I > guess a potential deadlock between process-A and process-B would happen in > such a executing sequence: > > (1) process-B got many journal-blocks, then came into ext3_new_blocks(), > hung up > (2) process-A locked i_mutex of the inode of a quota-file, then try to > starts a handle. Unfortunately, there are no enough journal-blocks left for > process-A. > (3) process-B awakened, and came into DQUOT_ALLOC_BLOCK(), finally came into > the function ext3_quota_write() who also wants to lock the i_mutex of the > inode of a quota-file. But the i_mutex was locked by process-A. so process-B > has no choice but to wait. > (4) if the ext3-filesystem was too busy to release jounal-blocks for > process-A, or a unexpected incident happened. Both the two situations would > result in no journal-blocks for any other processes. Apparently, process-A > have to wait for available journal-blocks. so process-A was hung-up with > i_mutex of the inode of a quota-file locked. > (5) process-B was blocked by the "inode->i_mutex" subsequently. Yes, that is a lock inversion between the journal lock and i_mutex on quota files which can indeed lead to a deadlock. Thanks for spotting it. Luckily it's not very likely you're going to hit it but we should fix it anyway. Currently I don't have much idea how - probably we'd have to get rid of the need to use i_mutex on quota files in quota_write but that's also non-trivial. Honza -- Jan Kara <jack@xxxxxxx> SuSE CR Labs - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
