Re: [PATCH v2 block/for-linus] writeback: flush inode cgroup wb switches instead of pinning super_block

[Jens Axboe]

On 02/29/2016 04:28 PM, Tejun Heo wrote:
> If cgroup writeback is in use, inodes can be scheduled for
> asynchronous wb switching.  Before 5ff8eaac1636 ("writeback: keep
> superblock pinned during cgroup writeback association switches"), this
> could race with umount leading to super_block being destroyed while
> inodes are pinned for wb switching.  5ff8eaac1636 fixed it by bumping
> s_active while wb switches are in flight; however, this allowed
> in-flight wb switches to make umounts asynchronous when the userland
> expected synchronosity - e.g. fsck immediately following umount may
> fail because the device is still busy.
>
> This patch removes the problematic super_block pinning and instead
> makes generic_shutdown_super() flush in-flight wb switches.  wb
> switches are now executed on a dedicated isw_wq so that they can be
> flushed and isw_nr_in_flight keeps track of the number of in-flight wb
> switches so that flushing can be avoided in most cases.
>
> v2: Move cgroup_writeback_umount() further below and add MS_ACTIVE
>      check in inode_switch_wbs() as Jan an Al suggested.

I queued this up for 4.5, but I'm feeling a bit uneasy about it. But 
it's either that, or revert 5ff8eaac1636 and fix it for real in 4.6. 
Jan/Tejun, what do you think?

--
Jens Axboe