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SubjectRe: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork
Hello, Aleksa.

On Wed, Apr 01, 2015 at 07:57:20PM +1100, Aleksa Sarai wrote:
> +struct cgroup_fork_state {
> + void *ss_state[CGROUP_SUBSYS_COUNT];
> +};

Can we collect the subsystems which require pre/post fork callbacks to
the front in group_subsys.h and do do CGROUP_SUBSYS_FORK_COUNT (or
whatever) instead? Then, we don't need all these subsys bitmasks
either we can just test the index against that and be done with it.

> +
> +/**
> + * cgroup_cfs_alloc - allocates an empty cgroup_fork_state
> + */
> +struct cgroup_fork_state *cgroup_cfs_alloc(void)
> +{
> + struct cgroup_fork_state *cfs;
> +
> + cfs = kzalloc(sizeof(struct cgroup_fork_state), GFP_KERNEL);
> + if (!cfs)
> + return ERR_PTR(-ENOMEM);
> +
> + return cfs;
> +}

Just make it a void * array and put it on stack. Abstraction at this
level doesn't serve any purpose. No controller code is gonna see this
anyway.

> +int cgroup_can_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
> +{
> + struct cgroup_subsys *ss;
> + int i, j, retval;
> +
> + for_each_subsys_which(need_canfork_callback, ss, i) {
> + retval = ss->can_fork(child, &cfs->ss_state[i]);
> + if (retval)
> + goto out_revert;
> + }
> +
> + return 0;
> +
> +out_revert:
> + for_each_subsys_which(need_cancelfork_callback, ss, j) {
> + if (j >= i)
> + break;
> + ss->cancel_fork(child, &cfs->ss_state[i]);

cancel_fork() has no reason to update the opaque pointer. No reason
to pass pointer of it.

> +void cgroup_cancel_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
> +{
> + struct cgroup_subsys *ss;
> + int i;
> +
> + for_each_subsys_which(need_cancelfork_callback, ss, i) {
> + void **state = NULL;
> +
> + /*
> + * Only if %ss has a can_fork() callback is %cfs->ss_state[i] meaningful

I don't think we do %var, do we? % is used for macros and consts.

> + * -- otherwise just pass a NULL.
> + */
> + if (need_canfork_callback & (1 << i))
> + state = &cfs->ss_state[i];
> +
> + ss->cancel_fork(child, &cfs->ss_state[i]);

Ditto, just pass the pointer itself.

> @@ -5241,8 +5346,18 @@ void cgroup_post_fork(struct task_struct *child)
> * css_set; otherwise, @child might change state between ->fork()
> * and addition to css_set.
> */
> - for_each_subsys_which(need_fork_callback, ss, i)
> - ss->fork(child);
> + for_each_subsys_which(need_fork_callback, ss, i) {
> + void **state = NULL;
> +
> + /*
> + * Only if %ss has a can_fork() callback is %old_cfs->ss_state[i]
> + * meaningful -- otherwise just pass a NULL.
> + */

Again, if you just passed the pointer, you wouldn't need the above.
Just clear the array on init and pass in whatever value is in there.

> + if (need_canfork_callback & (1 << i))
> + state = &old_cfs->ss_state[i];
> +
> + ss->fork(child, state);
> + }
> }

Thanks.

--
tejun


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