Re: [PATCH 1/2] sched/cgroup: move sched_online_group() back into css_online()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 26, 2017 at 12:41:41PM +0300, Konstantin Khlebnikov wrote:
> Commit 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init") moved
> sched_online_group() from css_online() to css_alloc(). It exposes half-baked
> task group into global lists before initializing generic cgroup stuff.
> 
> LTP testcase (third in cgroup_regression_test) written for testing
> similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> 

So nobody's run LTP against the kernel for almost a year?

> 
> Here task group already linked into global RCU-protected list task_groups
> but pointer css->cgroup is still NULL.
> 
> This patch reverts this chunk and moves online back to css_online().

Maybe put a comment with it that explains why this is needed?

Something along the lines of this perhaps?

/*
 * Don't expose the cgroup until initialization of it is complete in the
 * cgroup core. Otherwise things like cgroup_path() will return NULL
 * pointers and the like.
 */

> +static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
> +{
> +	struct task_group *tg = css_tg(css);
> +	struct task_group *parent = css_tg(css->parent);
> +
> +	if (parent)
> +		sched_online_group(tg, parent);
> +	return 0;
> +}