Re: [PATCH] writeback: inode cgroup wb switch should skip inode with zero i_count

[Tejun Heo]

On Wed, Jun 08, 2016 at 07:59:28PM -0700, Tahsin Erdogan wrote:
> Asynchronous wb switching of inodes takes an additional ref count on an
> inode to make sure inode remains valid until switchover is completed.
> 
> However, it is possible that inode->i_count has already reached zero
> while inode is in writeback queue:
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 917 at fs/inode.c:397 ihold+0x2b/0x30
> CPU: 1 PID: 917 Comm: kworker/u4:5 Not tainted 4.7.0-rc2+ #49
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
> 01/01/2011
> Workqueue: writeback wb_workfn (flush-8:16)
>  0000000000000000 ffff88007ca0fb58 ffffffff805990af 0000000000000000
>  0000000000000000 ffff88007ca0fb98 ffffffff80268702 0000018d000004e2
>  ffff88007cef40e8 ffff88007c9b89a8 ffff880079e3a740 0000000000000003
> Call Trace:
>  [<ffffffff805990af>] dump_stack+0x4d/0x6e
>  [<ffffffff80268702>] __warn+0xc2/0xe0
>  [<ffffffff802687d8>] warn_slowpath_null+0x18/0x20
>  [<ffffffff8035b4ab>] ihold+0x2b/0x30
>  [<ffffffff80367ecc>] inode_switch_wbs+0x11c/0x180
>  [<ffffffff80369110>] wbc_detach_inode+0x170/0x1a0
>  [<ffffffff80369abc>] writeback_sb_inodes+0x21c/0x530
>  [<ffffffff80369f7e>] wb_writeback+0xee/0x1e0
>  [<ffffffff8036a147>] wb_workfn+0xd7/0x280
>  [<ffffffff80287531>] ? try_to_wake_up+0x1b1/0x2b0
>  [<ffffffff8027bb09>] process_one_work+0x129/0x300
>  [<ffffffff8027be06>] worker_thread+0x126/0x480
>  [<ffffffff8098cde7>] ? __schedule+0x1c7/0x561
>  [<ffffffff8027bce0>] ? process_one_work+0x300/0x300
>  [<ffffffff80280ff4>] kthread+0xc4/0xe0
>  [<ffffffff80335578>] ? kfree+0xc8/0x100
>  [<ffffffff809903cf>] ret_from_fork+0x1f/0x40
>  [<ffffffff80280f30>] ? __kthread_parkme+0x70/0x70
> ---[ end trace aaefd2fd9f306bc4 ]---
> 
> Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Can you please repost the patch to Jens Axboe <axboe@xxxxxxxxx> with
acked-by added?  Please also cc Jan Kara <jack@xxxxxxxx>.

Thanks.

-- 
tejun