Re: [PATCH 02/15] writeback: update dirtied_when for synced inode to prevent livelock

Andrew Morton
2011-06-07T23:03:32Z

On Wed, 08 Jun 2011 05:32:38 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> Explicitly update .dirtied_when on synced inodes, so that they are no
> longer considered for writeback in the next round.

It sounds like this somewhat answers my questions for [1/15].

But I'm not seeing a description of exactly what caused the livelock.

> We'll do more aggressive "keep writeback as long as we wrote something"
> logic in wb_writeback(). The "use LONG_MAX .nr_to_write" trick in commit
> b9543dac5bbc ("writeback: avoid livelocking WB_SYNC_ALL writeback") will
> no longer be enough to stop sync livelock.
>
> It can prevent both of the following livelock schemes:
>
> - while true; do echo data >> f; done
> - while true; do touch f; done

You're kidding. This livelocks sync(1)? When did we break this?

Why is this? Because the inode keeps on getting rotated to head-of-list?