On Thu, 23 Oct 2014, Chris Friesen wrote:

On 10/17/2014 12:55 PM, Austin Schuh wrote:

quoted

Use the 121 patch.  This sounds very similar to the issue that I helped
debug with XFS.  There ended up being a deadlock due to a bug in the
kernel work queues.  You can search the RT archives for more info.

I can confirm that the problem still shows up with the rt121 patch. (And
also with Paul Gortmaker's proposed 3.4.103-rt127 patch.)

We added some instrumentation and it looks like we've tracked down the problem.
Figuring out how to fix it is proving to be tricky.

Basically it looks like we have a circular dependency involving the
inode->i_data_sem rt_mutex, the PG_writeback bit, and the BJ_Shadow list.  It
goes something like this:

jbd2_journal_commit_transaction:
1) set page for writeback (set PG_writeback bit)
2) put jbd2 journal head on BJ_Shadow list
3) sleep on PG_writeback bit waiting for page writeback complete

ext4_da_writepages:
1) ext4_map_blocks() acquires inode->i_data_sem for writing
2) do_get_write_access() sleeps waiting for jbd2 journal head to come off
the BJ_Shadow list

At this point the flush code can't run because it can't acquire
inode->i_data_sem for reading, so the page will never get written out.
Deadlock.

Sorry, I really cannot map that sparse description to any code
flow. Proper callchains for the involved parts might help to actually
understand what you are looking for.
 
Thanks,

	tglx