Re: [PATCH] use rcu_barrier() to wait for bdev puts at unmount

[Chris Mason <chris.mason@fusionio.com>]

On Sat, Mar 09, 2013 at 07:17:04AM -0700, Eric Sandeen wrote:
> On 3/9/13 6:27 AM, Chris Mason wrote:
> > On Fri, Mar 08, 2013 at 10:23:01PM -0700, Eric Sandeen wrote:
> >> Doing this would reliably fail with -EBUSY for me:
> >>
> >> # mount /dev/sdb2 /mnt/scratch; umount /mnt/scratch; mkfs.btrfs -f /dev/sdb2
> >> ...
> >> unable to open /dev/sdb2: Device or resource busy
> >>
> >> because mkfs.btrfs tries to open the device O_EXCL, and somebody still has it.
> >>
> >> Using systemtap to track bdev gets & puts shows a kworker thread doing a
> >> blkdev put after mkfs attempts a get; this is left over from the unmount.
> >>
> >> Adding an rcu_barrier() to btrfs_close_devices() causes unmount to wait
> >> until all blkdev_put()s are done, and the device is truly free once
> >> unmount completes.
> > 
> > Thanks for tracking this down Eric.  
> 
> Sure thing, sorry it took so long.
> 
> > Is this kworker triggered by btrfs
> > or is this something we should be doing for the other filesystems too?
> 
> It's all btrfs ;)
> 
> btrfs_close_devices
> 	__btrfs_close_devices
> 		call_rcu(&device->rcu, free_device);
> 			free_device
> 				INIT_WORK(&device->rcu_work, __free_device);
> 				schedule_work(&device->rcu_work);
> 

Great, that makes a ton more sense.  I'm a little confused on why we're
seeing it so much more now than in the past.

> 
> The behavior came from:
> 
> commit 1f78160ce1b1b8e657e2248118c4d91f881763f0
> Author: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> Date:   Wed Apr 20 10:09:16 2011 +0000
> 
>     Btrfs: using rcu lock in the reader side of devices list
> 
> Anyway, I can send V2 in close_ctree if you like. Thinking about it more
> though, btrfs_close_devices is closer to the action, so now I think
> I'd leave it there. :)
> 
> I probably should have put a comment in to say what the heck it's for,
> too.  Feel free to fix on merge or I can send another patch.

Ok, please add the comment and a cc to stable.  Thanks again.

-chris