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Message-Id: <AA05E959-86D4-42F4-8589-0C244FFB7BBB@dilger.ca> Date: Mon, 15 Jun 2015 11:59:59 -0600 From: Andreas Dilger <adilger@...ger.ca> To: Theodore Ts'o <tytso@....edu> Cc: Jan Kara <jack@...e.cz>, Ext4 Developers List <linux-ext4@...r.kernel.org>, enwlinux@...il.com, stable@...r.kernel.org Subject: Re: [PATCH] ext4: fix race between truncate and __ext4_journalled_writepage() On Jun 15, 2015, at 7:06 AM, Theodore Ts'o <tytso@....edu> wrote: > > On Mon, Jun 15, 2015 at 02:33:52PM +0200, Jan Kara wrote: >> Yeah, that's nasty. Thanks for debugging this! However I think your fix >> reintroduces the original deadlock issues. do_journal_get_write_access() >> can end up blocking waiting for jbd2 thread to finish a commit while jbd2 >> thread may be blocked waiting for the page to be unlocked. >> >> After some thought I don't think the deadlock is real since >> do_journal_get_write_access() will currently only block if a buffer is >> under writeout to the journal and at that point we don't wait for page >> locks anymore. Also ext4_write_begin() does the same in data=journal mode >> and we haven't observed deadlocks so far. But still things look really >> fragile here. > > The reason why there are no deadlocks is the writeback in the commit > thread happens when the inode gets written back --- but that only > happens for data=ordered inodes, not data=journalled mode. I was a > little worried about what might happen when after the 'j' chattr > attribute gets set on an inode, and the inode was still on the ordered > flush list. > > Hmm... I think we could also maybe fix this by having > ext4_change_inode_journal_flag() force a journal commit before setting > the JOURNAL_DATA flag. If we did that, we could just avoid dropping > the page_lock in __ext4_journalled_writepage() altogether. That is already being done: int ext4_change_inode_journal_flag(struct inode *inode, int val) { : : jbd2_journal_lock_updates(journal); : : /** * void jbd2_journal_lock_updates () - establish a transaction barrier. * @journal: Journal to establish a barrier on. * * This locks out any further updates from being started, and blocks * until all existing updates have completed, returning only once the * journal is in a quiescent state with no updates running. * * The journal lock should not be held on entry. */ Cheers, Andreas -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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