Re: [REGRESSION] cgroup: notify_on_release may not be triggered insome cases

From: Li Zefan
Date: Mon Oct 08 2012 - 02:13:06 EST


On 2012/10/4 15:37, Daisuke Nishimura wrote:
> notify_on_release must be triggered when the last process in a cgroup is
> move to another. But if the first(and only) process in a cgroup is moved to
> another, notify_on_release is not triggered.
>
> # mkdir /cgroup/cpu/SRC
> # mkdir /cgroup/cpu/DST
> #
> # echo 1 >/cgroup/cpu/SRC/notify_on_release
> # echo 1 >/cgroup/cpu/DST/notify_on_release
> #
> # sleep 300 &
> [1] 8629
> #
> # echo 8629 >/cgroup/cpu/SRC/tasks
> # echo 8629 >/cgroup/cpu/DST/tasks
> -> notify_on_release for /SRC must be triggered at this point,
> but it isn't.
>
> This is because put_css_set() is called before setting CGRP_RELEASABLE
> in cgroup_task_migrate(), and is a regression introduce by the
> commit:74a1166d(cgroups: make procs file writable), which was merged
> into v3.0.
>
> Cc: Ben Blum <bblum@xxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.0.x and later
> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> ---
> This patch is based on v3.6.
>
> kernel/cgroup.c | 3 +--
> 1 files changed, 1 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 7981850..7d01182 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1923,9 +1923,8 @@ static void cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
> * trading it for newcg is protected by cgroup_mutex, we're safe to drop
> * it here; it will be freed under RCU.
> */
> - put_css_set(oldcg);
> -
> set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
> + put_css_set(oldcg);

Good catch. Also we might end up with invalid memory access!

Acked-by: Li Zefan <lizefan@xxxxxxxxxx>

> }
>
> /**
>

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