Re: [PATCH 1/4] ext4: Fix deadlock during page writeback

[Theodore Ts'o <tytso@mit.edu>]

On Thu, Jun 16, 2016 at 12:42:13PM +0200, Jan Kara wrote:
> Commit 06bd3c36a733 (ext4: fix data exposure after a crash) uncovered a
> deadlock in ext4_writepages() which was previously much harder to hit.
> After this commit xfstest generic/130 reproduces the deadlock on small
> filesystems.
> 
> The problem happens when ext4_do_update_inode() sets LARGE_FILE feature
> and marks current inode handle as synchronous. That subsequently results
> in ext4_journal_stop() called from ext4_writepages() to block waiting for
> transaction commit while still holding page locks, reference to io_end,
> and some prepared bio in mpd structure each of which can possibly block
> transaction commit from completing and thus results in deadlock.

Would it be safe to submit the bio *after* calling
ext4_journal_stop()?  It looks like that would be safe, and I'd prefer
to minimize the time that a handle is open since that can really
impact performance when trying to close all existing handles when we
are starting commit processing.  It looks to me like this would be
safe in terms of avoiding deadlocks, yes?

						- Ted