Re: [PATCH] mmotm: fix hang at startup

[Wu Fengguang <fengguang.wu@intel.com>]

On Sun, May 01, 2011 at 10:35:38AM +0800, Hugh Dickins wrote:
> Yesterday's mmotm hangs at startup, and with lockdep it reports:
> BUG: spinlock recursion on CPU#1, blkid/284 - with bdi_lock_two()
> called from bdev_inode_switch_bdi() in the backtrace.  It appears
> that this function is sometimes called with new the same as old.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Thanks!

Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

> Fix to
> writeback-split-inode_wb_list_lock-into-bdi_writebacklist_lock.patch
> 
>  fs/block_dev.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> --- 2.6.39-rc5-mm1/fs/block_dev.c	2011-04-29 18:20:09.183314733 -0700
> +++ linux/fs/block_dev.c	2011-04-30 17:55:45.718785263 -0700
> @@ -57,6 +57,8 @@ static void bdev_inode_switch_bdi(struct
>  {
>  	struct backing_dev_info *old = inode->i_data.backing_dev_info;
>  
> +	if (dst == old)
> +		return;

nitpick: it could help to add a comment

        /* avoid spinlock recursion */

to indicate that's not merely an optional optimization, but indeed
required for correctness.

Thanks,
Fengguang

>  	bdi_lock_two(&old->wb, &dst->wb);
>  	spin_lock(&inode->i_lock);
>  	inode->i_data.backing_dev_info = dst;