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);
The behavior came from:
commit 1f78160ce1b1b8e657e2248118c4d91f881763f0
Author: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxx>
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.
Thanks,
-Eric
> I'd move it down to close_ctree, but I don't really have a good reason.
>
> -chris
>
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