Re: [PATCH -mmotm 3/4] memcg: avoid dead lock caused by race between oom and cpuset_attach

[Paul Menage <menage@google.com>]

On Sun, Dec 7, 2008 at 6:05 PM, Daisuke Nishimura
<nishimura@mxp.nes.nec.co.jp> wrote:
> mpol_rebind_mm(), which can be called from cpuset_attach(), does down_write(mm->mmap_sem).
> This means down_write(mm->mmap_sem) can be called under cgroup_mutex.
>
> OTOH, page fault path does down_read(mm->mmap_sem) and calls mem_cgroup_try_charge_xxx(),
> which may eventually calls mem_cgroup_out_of_memory(). And mem_cgroup_out_of_memory()
> calls cgroup_lock().
> This means cgroup_lock() can be called under down_read(mm->mmap_sem).

We should probably try to get cgroup_lock() out of the cpuset code
that calls mpol_rebind_mm() as well.

Paul

>
> If those two paths race, dead lock can happen.
>
> This patch avoid this dead lock by:
>  - remove cgroup_lock() from mem_cgroup_out_of_memory().
>  - define new mutex (memcg_tasklist) and serialize mem_cgroup_move_task()
>    (->attach handler of memory cgroup) and mem_cgroup_out_of_memory.
>
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu,com>
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> ---
>  mm/memcontrol.c |    5 +++++
>  mm/oom_kill.c   |    2 --
>  2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9877b03..fec4fc3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -51,6 +51,7 @@ static int really_do_swap_account __initdata = 1; /* for remember boot option*/
>  #define do_swap_account                (0)
>  #endif
>
> +static DEFINE_MUTEX(memcg_tasklist);   /* can be hold under cgroup_mutex */
>
>  /*
>  * Statistics for memory cgroup.
> @@ -797,7 +798,9 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
>
>                if (!nr_retries--) {
>                        if (oom) {
> +                               mutex_lock(&memcg_tasklist);
>                                mem_cgroup_out_of_memory(mem_over_limit, gfp_mask);
> +                               mutex_unlock(&memcg_tasklist);
>                                mem_over_limit->last_oom_jiffies = jiffies;
>                        }
>                        goto nomem;
> @@ -2173,10 +2176,12 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
>                                struct cgroup *old_cont,
>                                struct task_struct *p)
>  {
> +       mutex_lock(&memcg_tasklist);
>        /*
>         * FIXME: It's better to move charges of this process from old
>         * memcg to new memcg. But it's just on TODO-List now.
>         */
> +       mutex_unlock(&memcg_tasklist);
>  }
>
>  struct cgroup_subsys mem_cgroup_subsys = {
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index fd150e3..40ba050 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -429,7 +429,6 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
>        unsigned long points = 0;
>        struct task_struct *p;
>
> -       cgroup_lock();
>        read_lock(&tasklist_lock);
>  retry:
>        p = select_bad_process(&points, mem);
> @@ -444,7 +443,6 @@ retry:
>                goto retry;
>  out:
>        read_unlock(&tasklist_lock);
> -       cgroup_unlock();
>  }
>  #endif
>
>

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