Date:	Wed, 3 Jul 2013 18:08:13 +0400
From:	Glauber Costa <glommer@...il.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: slab shrinkers: BUG at mm/list_lru.c:92

On Wed, Jul 03, 2013 at 09:24:03PM +1000, Dave Chinner wrote:
> On Tue, Jul 02, 2013 at 02:44:27PM +0200, Michal Hocko wrote:
> > On Tue 02-07-13 22:19:47, Dave Chinner wrote:
> > [...]
> > > Ok, so it's been leaked from a dispose list somehow. Thanks for the
> > > info, Michal, it's time to go look at the code....
> > 
> > OK, just in case we will need it, I am keeping the machine in this state
> > for now. So we still can play with crash and check all the juicy
> > internals.
> 
> My current suspect is the LRU_RETRY code. I don't think what it is
> doing is at all valid - list_for_each_safe() is not safe if you drop
> the lock that protects the list. i.e. there is nothing that protects
> the stored next pointer from being removed from the list by someone
> else. Hence what I think is occurring is this:
> 
> 
> thread 1			thread 2
> lock(lru)
> list_for_each_safe(lru)		lock(lru)
>   isolate			......
>     lock(i_lock)
>     has buffers
>       __iget
>       unlock(i_lock)
>       unlock(lru)
>       .....			(gets lru lock)
>       				list_for_each_safe(lru)
> 				  walks all the inodes
> 				  finds inode being isolated by other thread
> 				  isolate
> 				    i_count > 0
> 				      list_del_init(i_lru)
> 				      return LRU_REMOVED;
> 				   moves to next inode, inode that
> 				   other thread has stored as next
> 				   isolate
> 				     i_state |= I_FREEING
> 				     list_move(dispose_list)
> 				     return LRU_REMOVED
> 				 ....
> 				 unlock(lru)
>       lock(lru)
>       return LRU_RETRY;
>   if (!first_pass)
>     ....
>   --nr_to_scan
>   (loop again using next, which has already been removed from the
>   LRU by the other thread!)
>   isolate
>     lock(i_lock)
>     if (i_state & ~I_REFERENCED)
>       list_del_init(i_lru)	<<<<< inode is on dispose list!
> 				<<<<< inode is now isolated, with I_FREEING set
>       return LRU_REMOVED;
> 
> That fits the corpse left on your machine, Michal. One thread has
> moved the inode to a dispose list, the other thread thinks it is
> still on the LRU and should be removed, and removes it.
> 
> This also explains the lru item count going negative - the same item
> is being removed from the lru twice. So it seems like all the
> problems you've been seeing are caused by this one problem....
> 
> Patch below that should fix this.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com
> 
> list_lru: fix broken LRU_RETRY behaviour
> 
> From: Dave Chinner <dchinner@...hat.com>
> 
> The LRU_RETRY code assumes that the list traversal status after we
> have dropped and regained the list lock. Unfortunately, this is not
> a valid assumption, and that can lead to racing traversals isolating
> objects that the other traversal expects to be the next item on the
> list.
> 
> This is causing problems with the inode cache shrinker isolation,
> with races resulting in an inode on a dispose list being "isolated"
> because a racing traversal still thinks it is on the LRU. The inode
> is then never reclaimed and that causes hangs if a subsequent lookup
> on that inode occurs.
> 
> Fix it by always restarting the list walk on a LRU_RETRY return from
> the isolate callback. Avoid the possibility of livelocks the current
> code was trying to aavoid by always decrementing the nr_to_walk
> counter on retries so that even if we keep hitting the same item on
> the list we'll eventually stop trying to walk and exit out of the
> situation causing the problem.
> 
> Reported-by: Michal Hocko <mhocko@...e.cz>
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> ---
>  mm/list_lru.c |   29 ++++++++++++-----------------
>  1 file changed, 12 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index dc71659..7246791 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -71,19 +71,19 @@ list_lru_walk_node(struct list_lru *lru, int nid, list_lru_walk_cb isolate,
>  	struct list_lru_node	*nlru = &lru->node[nid];
>  	struct list_head *item, *n;
>  	unsigned long isolated = 0;
> -	/*
> -	 * If we don't keep state of at which pass we are, we can loop at
> -	 * LRU_RETRY, since we have no guarantees that the caller will be able
> -	 * to do something other than retry on the next pass. We handle this by
> -	 * allowing at most one retry per object. This should not be altered
> -	 * by any condition other than LRU_RETRY.
> -	 */
> -	bool first_pass = true;
>  
>  	spin_lock(&nlru->lock);
>  restart:
>  	list_for_each_safe(item, n, &nlru->list) {
>  		enum lru_status ret;
> +
> +		/*
> +		 * decrement nr_to_walk first so that we don't livelock if we
> +		 * get stuck on large numbesr of LRU_RETRY items
> +		 */
> +		if (--(*nr_to_walk) == 0)
> +			break;
> +
>  		ret = isolate(item, &nlru->lock, cb_arg);
>  		switch (ret) {
>  		case LRU_REMOVED:
> @@ -98,19 +98,14 @@ restart:
>  		case LRU_SKIP:
>  			break;
>  		case LRU_RETRY:
> -			if (!first_pass) {
> -				first_pass = true;
> -				break;
> -			}
> -			first_pass = false;
> +			/*
> +			 * The lru lock has been dropped, our list traversal is
> +			 * now invalid and so we have to restart from scratch.
> +			 */
>  			goto restart;
>  		default:
>  			BUG();
>  		}
> -
> -		if ((*nr_to_walk)-- == 0)
> -			break;
> -
>  	}

This patch makes perfect sense to me, along with your description.

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