linux-kernel - Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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Date:   Fri, 15 Dec 2017 05:22:22 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Prateek Sood <prsood@...eaurora.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency

Hello, Prateek.

On Fri, Dec 15, 2017 at 02:24:55PM +0530, Prateek Sood wrote:
> Following are two ways to improve cgroup_transfer_tasks(). In
> both cases task in PF_EXITING state would be left in source
> cgroup. It would be removed from cgroup_exit() in exit path.
> 
> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
> index 024085d..e2bdcdb 100644
> --- a/kernel/cgroup/cgroup-v1.c
> +++ b/kernel/cgroup/cgroup-v1.c
> @@ -123,7 +123,10 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
>          */
>         do {
>                 css_task_iter_start(&from->self, 0, &it);
> -               task = css_task_iter_next(&it);
> +               do {
> +                       task = css_task_iter_next(&it);
> +               } while (task && (task & PF_EXITING))
> +

Yeah, this looks good to me.  We can't just make a single pass as in
the other one because we can race aginst fork.  And PF_EXITING being
left behind is what was happening previously too anyway.  They can't
be moved.

Thanks.

-- 
tejun