Re: Possible fix for lockup in drop_caches

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 17 Dec 2007 12:13:22 +0000 richard <richard@rsk.demon.co.uk> wrote:

>     fix lockup in when calling drop_caches
>     
>     calling /proc/sys/vm/drop_caches can hang due to a AB/BA lock dependency
>     between j_list_lock and the inode_lock. This patch moves the redirtying of the buffer head out
>     from under the j_list_lock.
>     
>     based on a suggestion by Andrew Morton.
>     

Oh boy.  Do we really want to add all this stuff to JBD just for
drop_caches which is a silly root-only broken-in-22-other-ways thing?

Michael, might your convert-inode-lists-to-tree patches eliminate the need
for taking inode_lock in drop_pagecache_sb()?  Probably not, as it uses an
rbtree.  It would have been possible if it was using a radix-tree, I
suspect..

> -void __journal_unfile_buffer(struct journal_head *jh)
> +void __journal_unfile_buffer(struct journal_head *jh,
> +		struct buffer_head **dirty_bh)
>  {
> -	__journal_temp_unlink_buffer(jh);
> +	__journal_temp_unlink_buffer(jh, dirty_bh);
>  	jh->b_transaction = NULL;
>  }

I suspect the code would end up simpler if __journal_unfile_buffer() were
to take an additional ref on the bh which it placed at *dirty_bh.

Callers of __journal_unfile_buffer() could then call

void handle_dirty_bh(struct buffer_head *bh)
{
	if (bh) {
		jbd_mark_buffer_dirty(bh);
		put_bh(bh);
	}
}

?