[PATCH] cgroups: don't put struct cgroupfs_root protected by RCU

[PATCH] cgroups: don't put struct cgroupfs_root protected by RCU

[Lai Jiangshan]

we don't access struct cgroupfs_root in fast path,
so we should not put struct cgroupfs_root protected by RCU

but the comment in struct cgroup_subsys.root confuse us.

struct cgroup_subsys.root is used in these places:

1 find_css_set(): if (ss->root->subsys_list.next == &ss->sibling)
2 rebind_subsystems(): if (ss->root != &rootnode)
                       rcu_assign_pointer(ss->root, root);
                       rcu_assign_pointer(subsys[i]->root, &rootnode);
3 cgroup_has_css_refs(): if (ss->root != cgrp->root)
4 cgroup_init_subsys(): ss->root = &rootnode;
5 proc_cgroupstats_show(): ss->name, ss->root->subsys_bits,
                           ss->root->number_of_cgroups, !ss->disabled);
6 cgroup_clone(): root = subsys->root;
                  if ((root != subsys->root) ||

all these place we have held cgroup_lock() or we don't dereference to
struct cgroupfs_root. It's means wo don't need RCU when use
struct cgroup_subsys.root, and we should not put struct cgroupfs_root
protected by RCU.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 1164963..22901ff 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -343,7 +345,6 @@ struct cgroup_subsys {
 #define MAX_CGROUP_TYPE_NAMELEN 32
 	const char *name;
 
-	/* Protected by RCU */
 	struct cgroupfs_root *root;
 
 	struct list_head sibling;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 358e775..ddc10ac 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -717,7 +722,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 			cgrp->subsys[i] = dummytop->subsys[i];
 			cgrp->subsys[i]->cgroup = cgrp;
 			list_add(&ss->sibling, &root->subsys_list);
-			rcu_assign_pointer(ss->root, root);
+			ss->root = root;
 			if (ss->bind)
 				ss->bind(ss, cgrp);
 
@@ -729,7 +734,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 				ss->bind(ss, dummytop);
 			dummytop->subsys[i]->cgroup = dummytop;
 			cgrp->subsys[i] = NULL;
-			rcu_assign_pointer(subsys[i]->root, &rootnode);
+			subsys[i]->root = &rootnode;
 			list_del(&ss->sibling);
 		} else if (bit & final_bits) {
 			/* Subsystem state should already exist */

Re: [PATCH] cgroups: don't put struct cgroupfs_root protected by RCU

[Paul Menage]

On Fri, Nov 21, 2008 at 12:49 AM, Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
>
> we don't access struct cgroupfs_root in fast path,
> so we should not put struct cgroupfs_root protected by RCU
>
> but the comment in struct cgroup_subsys.root confuse us.
>
> struct cgroup_subsys.root is used in these places:
>
> 1 find_css_set(): if (ss->root->subsys_list.next == &ss->sibling)
> 2 rebind_subsystems(): if (ss->root != &rootnode)
>                       rcu_assign_pointer(ss->root, root);
>                       rcu_assign_pointer(subsys[i]->root, &rootnode);
> 3 cgroup_has_css_refs(): if (ss->root != cgrp->root)
> 4 cgroup_init_subsys(): ss->root = &rootnode;
> 5 proc_cgroupstats_show(): ss->name, ss->root->subsys_bits,
>                           ss->root->number_of_cgroups, !ss->disabled);
> 6 cgroup_clone(): root = subsys->root;
>                  if ((root != subsys->root) ||
>
> all these place we have held cgroup_lock() or we don't dereference to
> struct cgroupfs_root. It's means wo don't need RCU when use
> struct cgroup_subsys.root, and we should not put struct cgroupfs_root
> protected by RCU.
>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>

Reviewed-by: Paul Menage <menage@google.com>

Thanks, seems reasonable.

> ---
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 1164963..22901ff 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -343,7 +345,6 @@ struct cgroup_subsys {
>  #define MAX_CGROUP_TYPE_NAMELEN 32
>        const char *name;
>
> -       /* Protected by RCU */
>        struct cgroupfs_root *root;
>
>        struct list_head sibling;
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 358e775..ddc10ac 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -717,7 +722,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
>                        cgrp->subsys[i] = dummytop->subsys[i];
>                        cgrp->subsys[i]->cgroup = cgrp;
>                        list_add(&ss->sibling, &root->subsys_list);
> -                       rcu_assign_pointer(ss->root, root);
> +                       ss->root = root;
>                        if (ss->bind)
>                                ss->bind(ss, cgrp);
>
> @@ -729,7 +734,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
>                                ss->bind(ss, dummytop);
>                        dummytop->subsys[i]->cgroup = dummytop;
>                        cgrp->subsys[i] = NULL;
> -                       rcu_assign_pointer(subsys[i]->root, &rootnode);
> +                       subsys[i]->root = &rootnode;
>                        list_del(&ss->sibling);
>                } else if (bit & final_bits) {
>                        /* Subsystem state should already exist */
>
>
>