[RFC][PATCH 1/8] cgroup: introduce cancel_attach()

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

This patch adds cancel_attach() operation to struct cgroup_subsys.
cancel_attach() can be used when can_attach() operation prepares something
for the subsys, but we should discard what can_attach() operation has prepared
if attach task/proc fails afterwards.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 Documentation/cgroups/cgroups.txt |   12 ++++++++++++
 include/linux/cgroup.h            |    2 ++
 kernel/cgroup.c                   |   36 ++++++++++++++++++++++++++++--------
 3 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 3df4b9a..07bb678 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -544,6 +544,18 @@ remain valid while the caller holds cgroup_mutex. 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.
+For example, this will be called if some subsystems are mounted on the same
+hierarchy, can_attach() operations have succeeded about part of the subsystems,
+but has failed about next subsystem. This will be called only about subsystems
+whose can_attach() operation has succeeded. This may be useful for subsystems
+which prepare something in can_attach() operation but should discard what has
+been prepared on failure.
+
 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 642a47f..a08edbc 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -429,6 +429,8 @@ 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 7da6004..2d9a808 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1700,7 +1700,7 @@ void threadgroup_fork_unlock(struct sighand_struct *sighand)
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
 	int retval;
-	struct cgroup_subsys *ss;
+	struct cgroup_subsys *ss, *fail = NULL;
 	struct cgroup *oldcgrp;
 	struct cgroupfs_root *root = cgrp->root;
 
@@ -1712,14 +1712,16 @@ 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) {
+				fail = ss;
+				goto out;
+			}
 		}
 	}
 
 	retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, 0);
 	if (retval)
-		return retval;
+		goto out;
 
 	for_each_subsys(root, ss) {
 		if (ss->attach)
@@ -1733,7 +1735,15 @@ 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 == fail)
+				break;
+			if (ss->cancel_attach)
+				ss->cancel_attach(ss, cgrp, tsk, false);
+		}
+	return retval;
 }
 
 /*
@@ -1813,7 +1823,7 @@ static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
 int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 {
 	int retval;
-	struct cgroup_subsys *ss;
+	struct cgroup_subsys *ss, *fail = NULL;
 	struct cgroup *oldcgrp;
 	struct css_set *oldcg;
 	struct cgroupfs_root *root = cgrp->root;
@@ -1839,8 +1849,10 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	for_each_subsys(root, ss) {
 		if (ss->can_attach) {
 			retval = ss->can_attach(ss, cgrp, leader, true);
-			if (retval)
-				return retval;
+			if (retval) {
+				fail = ss;
+				goto out;
+			}
 		}
 	}
 
@@ -1978,6 +1990,14 @@ list_teardown:
 		put_css_set(cg_entry->cg);
 		kfree(cg_entry);
 	}
+out:
+	if (retval)
+		for_each_subsys(root, ss) {
+			if (ss == fail)
+				break;
+			if (ss->cancel_attach)
+				ss->cancel_attach(ss, cgrp, tsk, true);
+		}
 	/* done! */
 	return retval;
 }
-- 
1.5.6.1

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