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>

3 files changed, 47 insertions, 8 deletions

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 0b33bfe7dde9..d45082653e3d 100644
--- 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 c9bbcb2a75ae..d08cfe7e12e5 100644
--- 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 4fd90e129772..be45d2f6008a 100644
--- 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;
 }
 
 /*