Re: [PATCH 1/2] memcg: fix endless loop in __mem_cgroup_iter_next

[Michal Hocko]

On Wed 12-02-14 17:26:46, Hugh Dickins wrote:
> Commit 0eef615665ed ("memcg: fix css reference leak and endless loop in
> mem_cgroup_iter") got the interaction with the commit a few before it
> d8ad30559715 ("mm/memcg: iteration skip memcgs not yet fully initialized")
> slightly wrong, and we didn't notice at the time.
> 
> It's elusive, and harder to get than the original, but for a couple of
> days before rc1, I several times saw a endless loop similar to that
> supposedly being fixed.
> 
> This time it was a tighter loop in __mem_cgroup_iter_next(): because we
> can get here when our root has already been offlined, and the ordering
> of conditions was such that we then just cycled around forever.
> 
> Fixes: 0eef615665ed ("memcg: fix css reference leak and endless loop in mem_cgroup_iter")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 3.12+

You are right of course. This is really embarrassing. I should have
noticed this when porting my original patch on top of yours.

Acked-by: Michal Hocko <mhocko@xxxxxxx>

Thanks!

> ---
> Of course I'd have preferred to send this before that commit went through
> to -stable, but priorities kept preempting; I did wonder whether to ask
> GregKH to delay it, but decided it's not serious enough to trouble him,
> just go with the flow of stable fixing stable.
> 
>  mm/memcontrol.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- 3.14-rc2/mm/memcontrol.c	2014-02-02 18:49:07.897302115 -0800
> +++ linux/mm/memcontrol.c	2014-02-12 11:55:02.836035004 -0800
> @@ -1127,8 +1127,8 @@ skip_node:
>  	 * skipping css reference should be safe.
>  	 */
>  	if (next_css) {
> -		if ((next_css->flags & CSS_ONLINE) &&
> -				(next_css == &root->css || css_tryget(next_css)))
> +		if ((next_css == &root->css) ||
> +		    ((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
>  			return mem_cgroup_from_css(next_css);
>  
>  		prev_css = next_css;

-- 
Michal Hocko
SUSE Labs
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