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Date: Wed, 4 Nov 2009 15:30:25 -0500
From: Ben Blum <bblum@...rew.cmu.edu>
To: linux-kernel@...r.kernel.org,
containers@...ts.linux-foundation.org, lizf@...fujistu.com,
akpm@...ux-foundation.org, menage@...gle.com, bblum@...rew.cmu.edu
Subject: Re: [RFC][PATCH 2/3] cgroups: subsystem module interface
On Mon, Nov 02, 2009 at 04:30:22PM -0500, Ben Blum wrote:
> Add interface between cgroups subsystem management and module loading
>
> From: Ben Blum <bblum@...rew.cmu.edu>
>
> This patch implements rudimentary module-loading support for cgroups - namely,
> a cgroup_load_subsys (similar to cgroup_init_subsys) for use as a module
> initcall, and a struct module pointer in struct cgroup_subsys. Support for
> subsystem unloading not here, yet to be done (requires more advanced reference
> counting, for one thing).
>
> I tested compiling a subsystem outside of the kernel source tree, and it was
> apparent that I wanted the EXPORT_SYMBOL for several functions, but I'm not
> sure what else it might be good to put them on. Suggestions welcome.
>
> Signed-off-by: Ben Blum <bblum@...rew.cmu.edu>
> ---
>
> include/linux/cgroup.h | 4 ++
> kernel/cgroup.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 96 insertions(+), 0 deletions(-)
>
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index d7f1545..c8474c4 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -38,6 +38,7 @@ extern void cgroup_fork_failed(struct task_struct *p, int run_callbacks,
> unsigned long clone_flags);
> extern int cgroupstats_build(struct cgroupstats *stats,
> struct dentry *dentry);
> +extern int cgroup_load_subsys(struct cgroup_subsys *ss);
>
> extern struct file_operations proc_cgroup_operations;
>
> @@ -477,6 +478,9 @@ struct cgroup_subsys {
> /* used when use_id == true */
> struct idr idr;
> spinlock_t id_lock;
> +
> + /* should be defined only by modular subsystems */
> + struct module *module;
> };
>
> #define SUBSYS(_x) extern struct cgroup_subsys _x ## _subsys;
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 9966dfe..8eba8a5 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2491,6 +2491,7 @@ int cgroup_add_file(struct cgroup *cgrp,
> error = PTR_ERR(dentry);
> return error;
> }
> +EXPORT_SYMBOL_GPL(cgroup_add_file);
>
> int cgroup_add_files(struct cgroup *cgrp,
> struct cgroup_subsys *subsys,
> @@ -2505,6 +2506,7 @@ int cgroup_add_files(struct cgroup *cgrp,
> }
> return 0;
> }
> +EXPORT_SYMBOL_GPL(cgroup_add_files);
>
> /**
> * cgroup_task_count - count the number of tasks in a cgroup.
> @@ -3650,7 +3652,97 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
> mutex_init(&ss->hierarchy_mutex);
> lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
> ss->active = 1;
> +
> + /* this function shouldn't be used with modular subsystems, since they
> + * need to register a subsys_id, among other things */
> + BUG_ON(ss->module);
> +}
> +
> +/**
> + * cgroup_load_subsys: load and register a modular subsystem at runtime
> + * @ss: the subsystem to load
> + *
> + * This function should be called in a modular subsystem's initcall. If the
> + * subsytem is built as a module, it will be assigned a new subsys_id and set
> + * up for use. If the subsystem is built-in anyway, work is delegated to the
> + * simpler cgroup_init_subsys.
> + */
> +int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
> +{
> + int i;
> + struct cgroup_subsys_state *css;
> +
> + /* check name validity */
> + if (ss->name == NULL || strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN)
> + return -EINVAL;
> +
> + /* we don't support callbacks in modular subsystems. this check is
> + * before the ss->module check for consistency - a module that *could*
> + * be a module should still have no callbacks for consistency. */
> + if (ss->fork || ss->exit)
> + return -EINVAL;
> +
> + /* an optionally modular subsystem is built-in: we want to do nothing,
> + * since cgroup_init_subsys will take care of it. */
> + if (ss->module == NULL) {
> + /* sanity: ss->module NULL only if the subsys is built-in and
> + * appears in subsys[] already. */
> + BUG_ON(ss->subsys_id >= CGROUP_BUILTIN_SUBSYS_COUNT);
> + BUG_ON(subsys[ss->subsys_id] != ss);
> + return 0;
> + }
> +
> + /* need to register a subsys id before anything else - for example,
> + * init_cgroup_css needs it. also, subsys_mutex needs to nest outside
> + * cgroup_mutex. */
> + down_write(&subsys_mutex);
> + /* find the first empty slot in the array */
> + for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
> + if (subsys[i] == NULL)
> + break;
> + }
> + if (i == CGROUP_SUBSYS_COUNT) {
> + /* maximum number of subsystems already registered! */
> + up_write(&subsys_mutex);
> + return -EBUSY;
> + }
> + /* assign ourselves the subsys_id */
> + ss->subsys_id = i;
> + subsys[i] = ss;
> +
> + mutex_lock(&cgroup_mutex);
> + /* no ss->create seems to need anything important in the ss struct, so
> + * this can happen first (i.e. before the rootnode attachment). */
> + css = ss->create(ss, dummytop);
> + if (IS_ERR(css)) {
> + /* failure case - need to deassign the subsys[] slot. */
> + mutex_unlock(&cgroup_mutex);
> + subsys[i] = NULL;
> + up_write(&subsys_mutex);
> + return PTR_ERR(css);
> + }
> +
> + list_add(&ss->sibling, &rootnode.subsys_list);
> + ss->root = &rootnode;
> +
> + init_cgroup_css(css, ss, dummytop);
> + init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
There is a problem here; namely, the init_css_set is supposed to be
immutable after boot. Putting a new subsystem state in it seems like it
will be impossible to get right, since a hierarchy might be mounted and
already using the init_css_set. Perhaps it would be better to use
find_css_set for this?
> +
> + mutex_init(&ss->hierarchy_mutex);
> + lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
> + ss->active = 1;
> +
> + /* pin the subsystem's module so it doesn't go away. this shouldn't
> + * fail, since the module's initcall calls us.
> + * TODO: with module unloading, move this elsewhere */
> + BUG_ON(!try_module_get(ss->module));
> +
> + /* success! */
> + mutex_unlock(&cgroup_mutex);
> + up_write(&subsys_mutex);
> + return 0;
> }
> +EXPORT_SYMBOL_GPL(cgroup_load_subsys);
>
> /**
> * cgroup_init_early - cgroup initialization at system boot
>
--
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