[PATCH v2] cgroup: remove redundate get/put of old css_set from migrate

[PATCH v2] cgroup: remove redundate get/put of old css_set from migrate

[Mandeep Singh Baines]

Changes in V2:
* Updated commit message as per Tejun's feedback:
  * https://lkml.org/lkml/2011/12/19/289

-- >8 -- (snip)

We can now assume that the css_set reference held by the task
will not go away for an exiting task. PF_EXITING state can be
trusted throughout migration by checking it after locking
threadgroup.

While at it, renamed css_set_check_fetched to css_set_fetched.
!css_set_fetched() seems to read better than
!css_set_check_fetched().

This patch depends on:

commit cd3d095275374220921fcf0d4e0c16584b26ddbc
Author: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Date:   Mon Dec 12 18:12:21 2011 -0800

    cgroup: always lock threadgroup during migration

Signed-off-by: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
---
 kernel/cgroup.c |   25 ++++++-------------------
 1 files changed, 6 insertions(+), 19 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1b3b841..eb95e32 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1856,7 +1856,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	 */
 	task_lock(tsk);
 	oldcg = tsk->cgroups;
-	get_css_set(oldcg);
 	task_unlock(tsk);
 
 	/* locate or allocate a new css_set for this task. */
@@ -1872,12 +1871,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 		might_sleep();
 		/* find_css_set will give us newcg already referenced. */
 		newcg = find_css_set(oldcg, cgrp);
-		if (!newcg) {
-			put_css_set(oldcg);
+		if (!newcg)
 			return -ENOMEM;
-		}
 	}
-	put_css_set(oldcg);
 
 	/* @tsk can't exit as its threadgroup is locked */
 	task_lock(tsk);
@@ -2015,9 +2011,8 @@ struct cg_list_entry {
 	struct list_head links;
 };
 
-static bool css_set_check_fetched(struct cgroup *cgrp,
-				  struct task_struct *tsk, struct css_set *cg,
-				  struct list_head *newcg_list)
+static bool css_set_fetched(struct cgroup *cgrp, struct task_struct *tsk,
+			    struct css_set *cg, struct list_head *newcg_list)
 {
 	struct css_set *newcg;
 	struct cg_list_entry *cg_entry;
@@ -2191,19 +2186,11 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		/* get old css_set pointer */
 		task_lock(tc->task);
 		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. */
-			put_css_set(oldcg);
-		} else {
-			/* we don't already have it. get new one. */
-			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
-			put_css_set(oldcg);
-			if (retval)
+		/* if we don't already have it in the list get a new one */
+		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
+			if (css_set_prefetch(cgrp, oldcg, &newcg_list))
 				goto out_list_teardown;
-		}
 	}
 
 	/*
-- 
1.7.3.1

Re: [PATCH v2] cgroup: remove redundate get/put of old css_set from migrate

[Frederic Weisbecker]

On Mon, Dec 19, 2011 at 10:44:34AM -0800, Mandeep Singh Baines wrote:
> Changes in V2:
> * Updated commit message as per Tejun's feedback:
>   * https://lkml.org/lkml/2011/12/19/289
> 
> -- >8 -- (snip)
> 
> We can now assume that the css_set reference held by the task
> will not go away for an exiting task. PF_EXITING state can be
> trusted throughout migration by checking it after locking
> threadgroup.
> 
> While at it, renamed css_set_check_fetched to css_set_fetched.
> !css_set_fetched() seems to read better than
> !css_set_check_fetched().
> 
> This patch depends on:
> 
> commit cd3d095275374220921fcf0d4e0c16584b26ddbc
> Author: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Date:   Mon Dec 12 18:12:21 2011 -0800
> 
>     cgroup: always lock threadgroup during migration
> 
> Signed-off-by: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
> Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> Cc: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
> ---
>  kernel/cgroup.c |   25 ++++++-------------------
>  1 files changed, 6 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1b3b841..eb95e32 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1856,7 +1856,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  	 */
>  	task_lock(tsk);
>  	oldcg = tsk->cgroups;
> -	get_css_set(oldcg);
>  	task_unlock(tsk);
>  
>  	/* locate or allocate a new css_set for this task. */
> @@ -1872,12 +1871,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  		might_sleep();
>  		/* find_css_set will give us newcg already referenced. */
>  		newcg = find_css_set(oldcg, cgrp);
> -		if (!newcg) {
> -			put_css_set(oldcg);
> +		if (!newcg)
>  			return -ENOMEM;
> -		}
>  	}
> -	put_css_set(oldcg);
>  
>  	/* @tsk can't exit as its threadgroup is locked */
>  	task_lock(tsk);
> @@ -2015,9 +2011,8 @@ struct cg_list_entry {
>  	struct list_head links;
>  };
>  
> -static bool css_set_check_fetched(struct cgroup *cgrp,
> -				  struct task_struct *tsk, struct css_set *cg,
> -				  struct list_head *newcg_list)
> +static bool css_set_fetched(struct cgroup *cgrp, struct task_struct *tsk,
> +			    struct css_set *cg, struct list_head *newcg_list)
>  {
>  	struct css_set *newcg;
>  	struct cg_list_entry *cg_entry;
> @@ -2191,19 +2186,11 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>  		/* get old css_set pointer */
>  		task_lock(tc->task);
>  		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. */
> -			put_css_set(oldcg);
> -		} else {
> -			/* we don't already have it. get new one. */
> -			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
> -			put_css_set(oldcg);
> -			if (retval)
> +		/* if we don't already have it in the list get a new one */
> +		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
> +			if (css_set_prefetch(cgrp, oldcg, &newcg_list))

Forgot to put the error value in retval?

>  				goto out_list_teardown;
> -		}
>  	}
>  
>  	/*
> -- 
> 1.7.3.1
> 

Thanks.

[PATCH 1/2] 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
---
 kernel/cgroup.c |   21 +++++++++++----------
 1 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5fdd98c..6b5f693 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1850,13 +1850,13 @@ 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;
-	task_unlock(tsk);
 
 	/* locate or allocate a new css_set for this task. */
 	if (guarantee) {
@@ -1875,9 +1875,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 			return -ENOMEM;
 	}
 
-	/* @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);
 
@@ -2177,10 +2175,13 @@ 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;
-		task_unlock(tc->task);
+
 		/* if we don't already have it in the list get a new one */
 		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
 			if (css_set_prefetch(cgrp, oldcg, &newcg_list))
-- 
1.7.5.4

[PATCH 2/2] 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
---
 kernel/cgroup.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6b5f693..a1e87a4 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4544,7 +4544,7 @@ 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
+ * it was not made under the protection of threadgroup_change_begin(), 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.
@@ -4554,10 +4554,14 @@ static const struct file_operations proc_cgroupstats_operations = {
  */
 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 1/2] cgroup: Remove unnecessary task_lock before fetching css_set on migration

[Tejun Heo]

Hello, Frederic.

On Tue, Dec 20, 2011 at 08:01:12PM +0100, Frederic Weisbecker wrote:
> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
> Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
> Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
> Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>

I like this but it doesn't apply on top of cgroup/for-3.3.  It
collides with Mandeep's patch.  Can you please rebase?

Li, can you please ack?

Thanks.

-- 
tejun

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

[Frederic Weisbecker]

On Tue, Dec 20, 2011 at 11:14:43AM -0800, Tejun Heo wrote:
> Hello, Frederic.
> 
> On Tue, Dec 20, 2011 at 08:01:12PM +0100, Frederic Weisbecker wrote:
> > 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> > Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> > Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
> > Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
> > Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> > Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> > Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
> > Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> 
> I like this but it doesn't apply on top of cgroup/for-3.3.  It
> collides with Mandeep's patch.  Can you please rebase?

Yeah it was made on top of last Mandeep's patch. Let me
rebase on top of your branch, will resend soon.

Thanks.

> 
> Li, can you please ack?
> 
> Thanks.
> 
> -- 
> tejun

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

[Mandeep Singh Baines]

Frederic Weisbecker (fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org) wrote:
> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
> Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
> Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
> Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>

Reviewed-by: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>

> ---
>  kernel/cgroup.c |   21 +++++++++++----------
>  1 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 5fdd98c..6b5f693 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1850,13 +1850,13 @@ 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;
> -	task_unlock(tsk);
>  
>  	/* locate or allocate a new css_set for this task. */
>  	if (guarantee) {
> @@ -1875,9 +1875,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  			return -ENOMEM;
>  	}
>  
> -	/* @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);
>  
> @@ -2177,10 +2175,13 @@ 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;
> -		task_unlock(tc->task);
> +
>  		/* if we don't already have it in the list get a new one */
>  		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
>  			if (css_set_prefetch(cgrp, oldcg, &newcg_list))
> -- 
> 1.7.5.4
> 

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

[Li Zefan]

Frederic Weisbecker wrote:
> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>

Good catch!

Reviewed-by: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>

> Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
> Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
> Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
> Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> ---
>  kernel/cgroup.c |   21 +++++++++++----------
>  1 files changed, 11 insertions(+), 10 deletions(-)