Hi everyone, Amir met a deadlock when he tested ext4 with snapshot. The deadlock was reported on https://github.com/amir73il/ext4-snapshots/commit/56396185d922a73524a091b545e665543abf741a. It is difficult to reproduce the deadlock. There is a deadlock reported on http://www.spinics.net/lists/linux-ext4/msg23018.html. Actually, these two deadlocks come from a same source. Below are my analysis on the 1st one. Mail is not a good place to describe parallel processes. I have submitted the analysis to https://github.com/YANGYongqiang/ext4-snapshots/blob/9e0ae9ae9907125e6bf45aa91db296d4cc041b17/fs/ext4/BUGS#L143. It is much more readable. -- deadlock in ext4 with snapshot ext4 with snapshot calls freeze_super() to bring a fs be in a clean state when a user takes a snapshot. freeze truncate kjournald | ext4_ext_truncate | freeze_super() | starts a handle | sets s_frozen | | | ext4_ext_truncate | | holds i_data_sem | ext4_freeze() | | commit_transaction() wait for updates | | waits for i_data_sem | ext4_free_blocks | | calls dquot_free_block| | | | dquot_free_block call | | ext4_dirty_inode | | | | ext4_dirty_inode | | trys to start a handle| | | | block due to s_frozen | in ext3, ext3_freeze() prevents journal from being updated by lock_journal_updates(), ext3_unfreeze() allow journal to be updated by unlock_journal_updates(). in ext4, however, before ext4_freeze() returns, it unlock journal, and ext4 prevents journal from being updated by s_frozen. s_frozen is in an upper layer, so it is out control of ext4 and deadlock is easy to happen. Could someone explain why ext4 does like above but not follow ext3? Yongqiang. -- Best Wishes Yongqiang Yang -- 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
