Re: [resend PATCH 3/3] writeback: fix false positive WARN in __mark_inode_dirty

[Dave Chinner <david@fromorbit.com>]

On Mon, Jan 04, 2016 at 10:20:16AM -0800, Dan Williams wrote:
> This warning was added as a debugging aid way back in commit
> 500b067c5e6c "writeback: check for registered bdi in flusher add and
> inode dirty" when we were switching over to per-bdi writeback.
> 
> Once the block device has been torn down it's no longer useful to
> complain about unregistered bdi's.  Clear the writeback capability under
> the the wb->list_lock(), so that __mark_inode_dirty has no opportunity
> to race bdi_unregister() to this WARN() condition.
> 
> Alternatively we could just delete the warning...

The warning is correct - the filesytem is trying to mark an inode
dirty on a device that can't do writeback anymore. Seems to me like
it is functioning as it should.

> Found this while testing block device remove from underneath an active
> fs triggering traces like:
> 
>  WARNING: CPU: 6 PID: 2129 at fs/fs-writeback.c:2065 __mark_inode_dirty+0x261/0x350()
>  bdi-block not registered
>  [..]
>  Call Trace:
>   [<ffffffff81459f02>] dump_stack+0x44/0x62
>   [<ffffffff810a1f32>] warn_slowpath_common+0x82/0xc0
>   [<ffffffff810a1fcc>] warn_slowpath_fmt+0x5c/0x80
>   [<ffffffff812830b1>] __mark_inode_dirty+0x261/0x350
>   [<ffffffff8126d019>] generic_update_time+0x79/0xd0
>   [<ffffffff8126d19d>] file_update_time+0xbd/0x110
>   [<ffffffff812e4ab8>] ext4_dax_fault+0x68/0x110
>   [<ffffffff811f7f3e>] __do_fault+0x4e/0xf0

This seems like the problem to me - you haven't implemented a
shutdown hook for ext4, and so it continues to allow page faults to
make progress after the device has been removed. The DAX fault
should have been failed before the filesystem gets to the point of
marking the inode dirty....

> +	/* tell __mark_inode_dirty that writeback is no longer possible */
> +	spin_lock(&wb->list_lock);
> +	wb->bdi->capabilities |= BDI_CAP_NO_WRITEBACK;
> +	spin_unlock(&wb->list_lock);
> +
>  	spin_unlock_bh(&wb->work_lock);

Is that lock ordering safe? i.e. it's inside a section using bh-safe
locking, which tends to imply that it can run from interrupt
contexts. Can we get something like

	spin_lock(&wb->list_lock);
	.....
	<irq>
	.....
	wb_shutdown
	spin_lock_bh(&wb->work_lock);
	spin_lock(&wb->list_lock);

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com