linux-kernel - Re: [PATCH] mmotm: fix hang at startup

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Message-ID: <20110501032507.GA21118@localhost>
Date:	Sun, 1 May 2011 11:25:07 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] mmotm: fix hang at startup

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@...gle.com>

Thanks!

Reviewed-by: Wu Fengguang <fengguang.wu@...el.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;
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