cgroups: add a per-subsystem hierarchy_mutex

These patches introduce new locking/refcount support for cgroups to
reduce the need for subsystems to call cgroup_lock(). This will
ultimately allow the atomicity of cgroup_rmdir() (which was removed
recently) to be restored.

These three patches give:

1/3 - introduce a per-subsystem hierarchy_mutex which a subsystem can
     use to prevent changes to its own cgroup tree

2/3 - use hierarchy_mutex in place of calling cgroup_lock() in the
     memory controller

3/3 - introduce a css_tryget() function similar to the one recently
      proposed by Kamezawa, but avoiding spurious refcount failures in
      the event of a race between a css_tryget() and an unsuccessful
      cgroup_rmdir()

Future patches will likely involve:

- using hierarchy mutex in place of cgroup_lock() in more subsystems
 where appropriate

- restoring the atomicity of cgroup_rmdir() with respect to cgroup_create()

This patch:

Add a hierarchy_mutex to the cgroup_subsys object that protects changes to
the hierarchy observed by that subsystem.  It is taken by the cgroup
subsystem (in addition to cgroup_mutex) for the following operations:

- linking a cgroup into that subsystem's cgroup tree
- unlinking a cgroup from that subsystem's cgroup tree
- moving the subsystem to/from a hierarchy (including across the
  bind() callback)

Thus if the subsystem holds its own hierarchy_mutex, it can safely
traverse its own hierarchy.

Signed-off-by: Paul Menage <menage@google.com>
Tested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 60287e9e9d2788e5af69678e8673bf4568e38056..e33ee74eee77000bc6a2df9ac437a0aa88632dcf 100644 (file)
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -528,7 +528,7 @@ example in cpusets, no task may attach before 'cpus' and 'mems' are set
 up.
 
 void bind(struct cgroup_subsys *ss, struct cgroup *root)
-(cgroup_mutex held by caller)
+(cgroup_mutex and ss->hierarchy_mutex held by caller)
 
 Called when a cgroup subsystem is rebound to a different hierarchy
 and root cgroup. Currently this will only involve movement between

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 73d1c730c3c417a318916ace4e2c4df6bed71237..ce1c1f34c30c81c310393959c793cfa6ad729538 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -340,8 +340,23 @@ struct cgroup_subsys {
 #define MAX_CGROUP_TYPE_NAMELEN 32
        const char *name;
 
-       struct cgroupfs_root *root;
+       /*
+        * Protects sibling/children links of cgroups in this
+        * hierarchy, plus protects which hierarchy (or none) the
+        * subsystem is a part of (i.e. root/sibling).  To avoid
+        * potential deadlocks, the following operations should not be
+        * undertaken while holding any hierarchy_mutex:
+        *
+        * - allocating memory
+        * - initiating hotplug events
+        */
+       struct mutex hierarchy_mutex;
 
+       /*
+        * Link to parent, and list entry in parent's children.
+        * Protected by this->hierarchy_mutex and cgroup_lock()
+        */
+       struct cgroupfs_root *root;
        struct list_head sibling;
 };
 

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 83ea4f524be545c12a83f2a7300ea08be0bd9960..8b6379cdf637fe5e6d5e55ac7915808c87dd55ea 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -722,23 +722,26 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                        BUG_ON(cgrp->subsys[i]);
                        BUG_ON(!dummytop->subsys[i]);
                        BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
+                       mutex_lock(&ss->hierarchy_mutex);
                        cgrp->subsys[i] = dummytop->subsys[i];
                        cgrp->subsys[i]->cgroup = cgrp;
                        list_move(&ss->sibling, &root->subsys_list);
                        ss->root = root;
                        if (ss->bind)
                                ss->bind(ss, cgrp);
-
+                       mutex_unlock(&ss->hierarchy_mutex);
                } else if (bit & removed_bits) {
                        /* We're removing this subsystem */
                        BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
                        BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
+                       mutex_lock(&ss->hierarchy_mutex);
                        if (ss->bind)
                                ss->bind(ss, dummytop);
                        dummytop->subsys[i]->cgroup = dummytop;
                        cgrp->subsys[i] = NULL;
                        subsys[i]->root = &rootnode;
                        list_move(&ss->sibling, &rootnode.subsys_list);
+                       mutex_unlock(&ss->hierarchy_mutex);
                } else if (bit & final_bits) {
                        /* Subsystem state should already exist */
                        BUG_ON(!cgrp->subsys[i]);
@@ -2338,6 +2341,29 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
        cgrp->subsys[ss->subsys_id] = css;
 }
 
+static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
+{
+       /* We need to take each hierarchy_mutex in a consistent order */
+       int i;
+
+       for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+               struct cgroup_subsys *ss = subsys[i];
+               if (ss->root == root)
+                       mutex_lock_nested(&ss->hierarchy_mutex, i);
+       }
+}
+
+static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
+{
+       int i;
+
+       for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+               struct cgroup_subsys *ss = subsys[i];
+               if (ss->root == root)
+                       mutex_unlock(&ss->hierarchy_mutex);
+       }
+}
+
 /*
  * cgroup_create - create a cgroup
  * @parent: cgroup that will be parent of the new cgroup
@@ -2386,7 +2412,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                init_cgroup_css(css, ss, cgrp);
        }
 
+       cgroup_lock_hierarchy(root);
        list_add(&cgrp->sibling, &cgrp->parent->children);
+       cgroup_unlock_hierarchy(root);
        root->number_of_cgroups++;
 
        err = cgroup_create_dir(cgrp, dentry, mode);
@@ -2504,8 +2532,12 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
        if (!list_empty(&cgrp->release_list))
                list_del(&cgrp->release_list);
        spin_unlock(&release_list_lock);
-       /* delete my sibling from parent->children */
+
+       cgroup_lock_hierarchy(cgrp->root);
+       /* delete this cgroup from parent->children */
        list_del(&cgrp->sibling);
+       cgroup_unlock_hierarchy(cgrp->root);
+
        spin_lock(&cgrp->dentry->d_lock);
        d = dget(cgrp->dentry);
        spin_unlock(&d->d_lock);
@@ -2547,6 +2579,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         * need to invoke fork callbacks here. */
        BUG_ON(!list_empty(&init_task.tasks));
 
+       mutex_init(&ss->hierarchy_mutex);
        ss->active = 1;
 }