Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned during cgroup writeback association switches

[Tejun Heo <tj@kernel.org>]

Hello, Jan.

On Thu, Feb 18, 2016 at 10:55:38AM +0100, Jan Kara wrote:
> I'm not sure I understand the question. Do you mean why both s_active and
> s_umount rwsem exist? s_active is a reference count keeping superblock

Yes.

> alive - e.g. if the filesystem is mounted in more places, we need a
> reference for each mountpoint. s_umount is used when we want to block any

I could be mistaken but I *think* we used to reject umounts based on
s_active and s_umount is the mechanism to delay umounts rather than
failing them and probably with bind mounts the behavior changed.

> umount operation until we are done. For example sync(2) is using it to make
> sure superblock doesn't disappear and so that we don't keep superblock
> alive after admin called umount(2).

So, the question is why aren't we just using s_active and draining it
on umount of the last mountpoint.  Because, right now, the behavior is
weird in that we allow umounts to proceed but then let the superblock
hang onto the block device till s_active is drained.  This really
should be synchronous.

Thanks.

-- 
tejun