[PATCH 0/2 v3] cgroup: Remove useless task_lock()

[PATCH 0/2 v3] cgroup: Remove useless task_lock()

[Frederic Weisbecker]

Hi,

I just updated the comments in the second patch to better
explain the locking involved, otherwise nothing new.

Thanks.

Frederic Weisbecker (2):
  cgroup: Remove unnecessary task_lock before fetching css_set on
    migration
  cgroup: Drop task_lock(parent) on cgroup_fork()

 kernel/cgroup.c |   43 +++++++++++++++++++++++++++----------------
 1 files changed, 27 insertions(+), 16 deletions(-)

-- 
1.7.5.4

[PATCH 1/2 v3] cgroup: Remove unnecessary task_lock before fetching css_set on migration

[Frederic Weisbecker]

When we fetch the css_set of the tasks on cgroup migration, we don't need
anymore to synchronize against cgroup_exit() that could swap the old one
with init_css_set. Now that we are using threadgroup_lock() during
the migrations, we don't need to worry about it anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <cgroups@vger.kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
---
 kernel/cgroup.c |   20 ++++++++++----------
 1 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index bc3caff..24f6d6f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1850,14 +1850,14 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	struct css_set *newcg;
 
 	/*
-	 * get old css_set. we need to take task_lock and refcount it, because
-	 * an exiting task can change its css_set to init_css_set and drop its
-	 * old one without taking cgroup_mutex.
+	 * get old css_set. We are synchronized through threadgroup_lock()
+	 * against PF_EXITING setting such that we can't race against
+	 * cgroup_exit() changing the css_set to init_css_set and dropping the
+	 * old one.
 	 */
-	task_lock(tsk);
+	WARN_ON_ONCE(tsk->flags & PF_EXITING);
 	oldcg = tsk->cgroups;
 	get_css_set(oldcg);
-	task_unlock(tsk);
 
 	/* locate or allocate a new css_set for this task. */
 	if (guarantee) {
@@ -1879,9 +1879,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	}
 	put_css_set(oldcg);
 
-	/* @tsk can't exit as its threadgroup is locked */
 	task_lock(tsk);
-	WARN_ON_ONCE(tsk->flags & PF_EXITING);
 	rcu_assign_pointer(tsk->cgroups, newcg);
 	task_unlock(tsk);
 
@@ -2182,11 +2180,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		/* nothing to do if this task is already in the cgroup */
 		if (tc->cgrp == cgrp)
 			continue;
-		/* get old css_set pointer */
-		task_lock(tc->task);
+		/*
+		 * get old css_set pointer. threadgroup is locked so this is
+		 * safe against concurrent cgroup_exit() changing this to
+		 * init_css_set.
+		 */
 		oldcg = tc->task->cgroups;
 		get_css_set(oldcg);
-		task_unlock(tc->task);
 		/* see if the new one for us is already in the list? */
 		if (css_set_check_fetched(cgrp, tc->task, oldcg, &newcg_list)) {
 			/* was already there, nothing to do. */
-- 
1.7.5.4

[PATCH 2/2 v3] cgroup: Drop task_lock(parent) on cgroup_fork()

[Frederic Weisbecker]

We don't need to hold the parent task_lock() on the
parent in cgroup_fork() because we are already synchronized
against the two places that may change the parent css_set
concurrently:

- cgroup_exit(), but the parent obviously can't exit concurrently
- cgroup migration: we are synchronized against threadgroup_lock()

So we can safely remove the task_lock() there.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <cgroups@vger.kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Mandeep Singh Baines <msb@chromium.org>
---
 kernel/cgroup.c |   23 +++++++++++++++++------
 1 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 24f6d6f..4936d88 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4556,20 +4556,31 @@ static const struct file_operations proc_cgroupstats_operations = {
  *
  * A pointer to the shared css_set was automatically copied in
  * fork.c by dup_task_struct().  However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
- * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
- * have already changed current->cgroups, allowing the previously
- * referenced cgroup group to be removed and freed.
+ * it was not made under the protection of RCU, cgroup_mutex or
+ * threadgroup_change_begin(), so it might no longer be a valid
+ * cgroup pointer.  cgroup_attach_task() might have already changed
+ * current->cgroups, allowing the previously referenced cgroup
+ * group to be removed and freed.
+ *
+ * Outside the pointer validity we also need to process the css_set
+ * inheritance between threadgoup_change_begin() and
+ * threadgoup_change_end(), this way there is no leak in any process
+ * wide migration performed by cgroup_attach_proc() that could otherwise
+ * miss a thread because it is too early or too late in the fork stage.
  *
  * At the point that cgroup_fork() is called, 'current' is the parent
  * task, and the passed argument 'child' points to the child task.
  */
 void cgroup_fork(struct task_struct *child)
 {
-	task_lock(current);
+	/*
+	 * We don't need to task_lock() current because current->cgroups
+	 * can't be changed concurrently here. The parent obviously hasn't
+	 * exited and called cgroup_exit(), and we are synchronized against
+	 * cgroup migration through threadgroup_change_begin().
+	 */
 	child->cgroups = current->cgroups;
 	get_css_set(child->cgroups);
-	task_unlock(current);
 	INIT_LIST_HEAD(&child->cg_list);
 }
 
-- 
1.7.5.4

Re: [PATCH 2/2 v3] cgroup: Drop task_lock(parent) on cgroup_fork()

[Tejun Heo]

On Wed, Dec 21, 2011 at 08:03:19PM +0100, Frederic Weisbecker wrote:
> We don't need to hold the parent task_lock() on the
> parent in cgroup_fork() because we are already synchronized
> against the two places that may change the parent css_set
> concurrently:
> 
> - cgroup_exit(), but the parent obviously can't exit concurrently
> - cgroup migration: we are synchronized against threadgroup_lock()
> 
> So we can safely remove the task_lock() there.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: Containers <containers@lists.linux-foundation.org>
> Cc: Cgroups <cgroups@vger.kernel.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> Cc: Mandeep Singh Baines <msb@chromium.org>

Applied both patches to cgroup/for-3.3 and pushed out to linux-next.

Thank you.

-- 
tejun