cgroup: introduce cancel_attach()

Add cancel_attach() operation to struct cgroup_subsys.  cancel_attach()
can be used when can_attach() operation prepares something for the subsys,
but we should rollback what can_attach() operation has prepared if attach
task fails after we've succeeded in can_attach().

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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 0b33bfe7dde99ecc98df6d0fa4466b3e79eb8f2b..d45082653e3df1800d213ba1210fce1f657c9bf9 100644 (file)
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -536,10 +536,21 @@ returns an error, this will abort the attach operation.  If a NULL
 task is passed, then a successful result indicates that *any*
 unspecified task can be moved into the cgroup. Note that this isn't
 called on a fork. If this method returns 0 (success) then this should
-remain valid while the caller holds cgroup_mutex. If threadgroup is
+remain valid while the caller holds cgroup_mutex and it is ensured that either
+attach() or cancel_attach() will be called in future. If threadgroup is
 true, then a successful result indicates that all threads in the given
 thread's threadgroup can be moved together.
 
+void cancel_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+              struct task_struct *task, bool threadgroup)
+(cgroup_mutex held by caller)
+
+Called when a task attach operation has failed after can_attach() has succeeded.
+A subsystem whose can_attach() has some side-effects should provide this
+function, so that the subsytem can implement a rollback. If not, not necessary.
+This will be called only about subsystems whose can_attach() operation have
+succeeded.
+
 void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
            struct cgroup *old_cgrp, struct task_struct *task,
            bool threadgroup)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c9bbcb2a75aecbacdb1463cf08a2b0a26d0f264b..d08cfe7e12e5f48036a84f6014fc983f052c2051 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -428,6 +428,8 @@ struct cgroup_subsys {
        void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
        int (*can_attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
                          struct task_struct *tsk, bool threadgroup);
+       void (*cancel_attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
+                         struct task_struct *tsk, bool threadgroup);
        void (*attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
                        struct cgroup *old_cgrp, struct task_struct *tsk,
                        bool threadgroup);

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 4fd90e12977236f54a4dd29168bc2db04bb1cdbc..be45d2f6008a1d2a2072a7e69118ae53656491b4 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1554,7 +1554,7 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
        int retval = 0;
-       struct cgroup_subsys *ss;
+       struct cgroup_subsys *ss, *failed_ss = NULL;
        struct cgroup *oldcgrp;
        struct css_set *cg;
        struct css_set *newcg;
@@ -1568,8 +1568,16 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
        for_each_subsys(root, ss) {
                if (ss->can_attach) {
                        retval = ss->can_attach(ss, cgrp, tsk, false);
-                       if (retval)
-                               return retval;
+                       if (retval) {
+                               /*
+                                * Remember on which subsystem the can_attach()
+                                * failed, so that we only call cancel_attach()
+                                * against the subsystems whose can_attach()
+                                * succeeded. (See below)
+                                */
+                               failed_ss = ss;
+                               goto out;
+                       }
                }
        }
 
@@ -1583,14 +1591,17 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
         */
        newcg = find_css_set(cg, cgrp);
        put_css_set(cg);
-       if (!newcg)
-               return -ENOMEM;
+       if (!newcg) {
+               retval = -ENOMEM;
+               goto out;
+       }
 
        task_lock(tsk);
        if (tsk->flags & PF_EXITING) {
                task_unlock(tsk);
                put_css_set(newcg);
-               return -ESRCH;
+               retval = -ESRCH;
+               goto out;
        }
        rcu_assign_pointer(tsk->cgroups, newcg);
        task_unlock(tsk);
@@ -1616,7 +1627,22 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
         * is no longer empty.
         */
        cgroup_wakeup_rmdir_waiter(cgrp);
-       return 0;
+out:
+       if (retval) {
+               for_each_subsys(root, ss) {
+                       if (ss == failed_ss)
+                               /*
+                                * This subsystem was the one that failed the
+                                * can_attach() check earlier, so we don't need
+                                * to call cancel_attach() against it or any
+                                * remaining subsystems.
+                                */
+                               break;
+                       if (ss->cancel_attach)
+                               ss->cancel_attach(ss, cgrp, tsk, false);
+               }
+       }
+       return retval;
 }
 
 /*