Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-08-03 Thread Greg KH 
On Wed, Jul 26, 2017 at 09:59:43PM +0100, Matt Fleming wrote:
> On Wed, 26 Jul, at 07:35:51AM, Greg KH wrote:
> > 
> > If it needs a backport and a simple cherry-pick does not work, yes
> > please.
> 
> Oh, it turns out cherry-picking commit 96b777452d88 to 4.9-stable
> works just fine.

Great, now queued up.

thanks,

greg k-h

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-08-03 Thread Greg KH 
On Wed, Jul 26, 2017 at 09:59:43PM +0100, Matt Fleming wrote:
> On Wed, 26 Jul, at 07:35:51AM, Greg KH wrote:
> > 
> > If it needs a backport and a simple cherry-pick does not work, yes
> > please.
> 
> Oh, it turns out cherry-picking commit 96b777452d88 to 4.9-stable
> works just fine.

Great, now queued up.

thanks,

greg k-h

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Matt Fleming 
On Wed, 26 Jul, at 07:35:51AM, Greg KH wrote:
> 
> If it needs a backport and a simple cherry-pick does not work, yes
> please.

Oh, it turns out cherry-picking commit 96b777452d88 to 4.9-stable
works just fine.

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Matt Fleming 
On Wed, 26 Jul, at 07:35:51AM, Greg KH wrote:
> 
> If it needs a backport and a simple cherry-pick does not work, yes
> please.

Oh, it turns out cherry-picking commit 96b777452d88 to 4.9-stable
works just fine.

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Greg KH 
On Wed, Jul 26, 2017 at 01:05:23PM +0100, Matt Fleming wrote:
> On Tue, 25 Jul, at 11:04:39AM, Greg KH wrote:
> > On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> > > From: Konstantin Khlebnikov 
> > > 
> > > commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> > > 
> > > Commit:
> > > 
> > >   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > 
> > > .. moved sched_online_group() from css_online() to css_alloc().
> > > It exposes half-baked task group into global lists before initializing
> > > generic cgroup stuff.
> > > 
> > > LTP testcase (third in cgroup_regression_test) written for testing
> > > similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> > > 
> > >   BUG: unable to handle kernel NULL pointer dereference at 
> > > 0008
> > >   IP: kernfs_path_from_node_locked+0x260/0x320
> > >   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
> > >   Call Trace:
> > >   ? kernfs_path_from_node+0x4f/0x60
> > >   kernfs_path_from_node+0x3e/0x60
> > >   print_rt_rq+0x44/0x2b0
> > >   print_rt_stats+0x7a/0xd0
> > >   print_cpu+0x2fc/0xe80
> > >   ? __might_sleep+0x4a/0x80
> > >   sched_debug_show+0x17/0x30
> > >   seq_read+0xf2/0x3b0
> > >   proc_reg_read+0x42/0x70
> > >   __vfs_read+0x28/0x130
> > >   ? security_file_permission+0x9b/0xc0
> > >   ? rw_verify_area+0x4e/0xb0
> > >   vfs_read+0xa5/0x170
> > >   SyS_read+0x46/0xa0
> > >   entry_SYSCALL_64_fastpath+0x1e/0xad
> > > 
> > > Here the task group is already linked into the global RCU-protected 
> > > 'task_groups'
> > > list, but the css->cgroup pointer is still NULL.
> > > 
> > > This patch reverts this chunk and moves online back to css_online().
> > > 
> > > Signed-off-by: Konstantin Khlebnikov 
> > > Signed-off-by: Peter Zijlstra (Intel) 
> > > Cc: Linus Torvalds 
> > > Cc: Peter Zijlstra 
> > > Cc: Tejun Heo 
> > > Cc: Thomas Gleixner 
> > > Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > Link: 
> > > http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> > > Signed-off-by: Ingo Molnar 
> > > Signed-off-by: Matt Fleming 
> > > ---
> > >  kernel/sched/core.c | 14 --
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > What about 4.9-stable, this should go there too, right?
> 
> Yes, good catch. Would you like me to send a separate patch?

If it needs a backport and a simple cherry-pick does not work, yes
please.

thanks,

greg k-h

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Greg KH 
On Wed, Jul 26, 2017 at 01:05:23PM +0100, Matt Fleming wrote:
> On Tue, 25 Jul, at 11:04:39AM, Greg KH wrote:
> > On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> > > From: Konstantin Khlebnikov 
> > > 
> > > commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> > > 
> > > Commit:
> > > 
> > >   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > 
> > > .. moved sched_online_group() from css_online() to css_alloc().
> > > It exposes half-baked task group into global lists before initializing
> > > generic cgroup stuff.
> > > 
> > > LTP testcase (third in cgroup_regression_test) written for testing
> > > similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> > > 
> > >   BUG: unable to handle kernel NULL pointer dereference at 
> > > 0008
> > >   IP: kernfs_path_from_node_locked+0x260/0x320
> > >   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
> > >   Call Trace:
> > >   ? kernfs_path_from_node+0x4f/0x60
> > >   kernfs_path_from_node+0x3e/0x60
> > >   print_rt_rq+0x44/0x2b0
> > >   print_rt_stats+0x7a/0xd0
> > >   print_cpu+0x2fc/0xe80
> > >   ? __might_sleep+0x4a/0x80
> > >   sched_debug_show+0x17/0x30
> > >   seq_read+0xf2/0x3b0
> > >   proc_reg_read+0x42/0x70
> > >   __vfs_read+0x28/0x130
> > >   ? security_file_permission+0x9b/0xc0
> > >   ? rw_verify_area+0x4e/0xb0
> > >   vfs_read+0xa5/0x170
> > >   SyS_read+0x46/0xa0
> > >   entry_SYSCALL_64_fastpath+0x1e/0xad
> > > 
> > > Here the task group is already linked into the global RCU-protected 
> > > 'task_groups'
> > > list, but the css->cgroup pointer is still NULL.
> > > 
> > > This patch reverts this chunk and moves online back to css_online().
> > > 
> > > Signed-off-by: Konstantin Khlebnikov 
> > > Signed-off-by: Peter Zijlstra (Intel) 
> > > Cc: Linus Torvalds 
> > > Cc: Peter Zijlstra 
> > > Cc: Tejun Heo 
> > > Cc: Thomas Gleixner 
> > > Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > > Link: 
> > > http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> > > Signed-off-by: Ingo Molnar 
> > > Signed-off-by: Matt Fleming 
> > > ---
> > >  kernel/sched/core.c | 14 --
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > What about 4.9-stable, this should go there too, right?
> 
> Yes, good catch. Would you like me to send a separate patch?

If it needs a backport and a simple cherry-pick does not work, yes
please.

thanks,

greg k-h

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Matt Fleming 
On Tue, 25 Jul, at 11:04:39AM, Greg KH wrote:
> On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> > From: Konstantin Khlebnikov 
> > 
> > commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> > 
> > Commit:
> > 
> >   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > 
> > .. moved sched_online_group() from css_online() to css_alloc().
> > It exposes half-baked task group into global lists before initializing
> > generic cgroup stuff.
> > 
> > LTP testcase (third in cgroup_regression_test) written for testing
> > similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> > 
> >   BUG: unable to handle kernel NULL pointer dereference at 0008
> >   IP: kernfs_path_from_node_locked+0x260/0x320
> >   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
> >   Call Trace:
> >   ? kernfs_path_from_node+0x4f/0x60
> >   kernfs_path_from_node+0x3e/0x60
> >   print_rt_rq+0x44/0x2b0
> >   print_rt_stats+0x7a/0xd0
> >   print_cpu+0x2fc/0xe80
> >   ? __might_sleep+0x4a/0x80
> >   sched_debug_show+0x17/0x30
> >   seq_read+0xf2/0x3b0
> >   proc_reg_read+0x42/0x70
> >   __vfs_read+0x28/0x130
> >   ? security_file_permission+0x9b/0xc0
> >   ? rw_verify_area+0x4e/0xb0
> >   vfs_read+0xa5/0x170
> >   SyS_read+0x46/0xa0
> >   entry_SYSCALL_64_fastpath+0x1e/0xad
> > 
> > Here the task group is already linked into the global RCU-protected 
> > 'task_groups'
> > list, but the css->cgroup pointer is still NULL.
> > 
> > This patch reverts this chunk and moves online back to css_online().
> > 
> > Signed-off-by: Konstantin Khlebnikov 
> > Signed-off-by: Peter Zijlstra (Intel) 
> > Cc: Linus Torvalds 
> > Cc: Peter Zijlstra 
> > Cc: Tejun Heo 
> > Cc: Thomas Gleixner 
> > Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > Link: 
> > http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> > Signed-off-by: Ingo Molnar 
> > Signed-off-by: Matt Fleming 
> > ---
> >  kernel/sched/core.c | 14 --
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> What about 4.9-stable, this should go there too, right?

Yes, good catch. Would you like me to send a separate patch?

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-26 Thread Matt Fleming 
On Tue, 25 Jul, at 11:04:39AM, Greg KH wrote:
> On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> > From: Konstantin Khlebnikov 
> > 
> > commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> > 
> > Commit:
> > 
> >   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > 
> > .. moved sched_online_group() from css_online() to css_alloc().
> > It exposes half-baked task group into global lists before initializing
> > generic cgroup stuff.
> > 
> > LTP testcase (third in cgroup_regression_test) written for testing
> > similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> > 
> >   BUG: unable to handle kernel NULL pointer dereference at 0008
> >   IP: kernfs_path_from_node_locked+0x260/0x320
> >   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
> >   Call Trace:
> >   ? kernfs_path_from_node+0x4f/0x60
> >   kernfs_path_from_node+0x3e/0x60
> >   print_rt_rq+0x44/0x2b0
> >   print_rt_stats+0x7a/0xd0
> >   print_cpu+0x2fc/0xe80
> >   ? __might_sleep+0x4a/0x80
> >   sched_debug_show+0x17/0x30
> >   seq_read+0xf2/0x3b0
> >   proc_reg_read+0x42/0x70
> >   __vfs_read+0x28/0x130
> >   ? security_file_permission+0x9b/0xc0
> >   ? rw_verify_area+0x4e/0xb0
> >   vfs_read+0xa5/0x170
> >   SyS_read+0x46/0xa0
> >   entry_SYSCALL_64_fastpath+0x1e/0xad
> > 
> > Here the task group is already linked into the global RCU-protected 
> > 'task_groups'
> > list, but the css->cgroup pointer is still NULL.
> > 
> > This patch reverts this chunk and moves online back to css_online().
> > 
> > Signed-off-by: Konstantin Khlebnikov 
> > Signed-off-by: Peter Zijlstra (Intel) 
> > Cc: Linus Torvalds 
> > Cc: Peter Zijlstra 
> > Cc: Tejun Heo 
> > Cc: Thomas Gleixner 
> > Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> > Link: 
> > http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> > Signed-off-by: Ingo Molnar 
> > Signed-off-by: Matt Fleming 
> > ---
> >  kernel/sched/core.c | 14 --
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> What about 4.9-stable, this should go there too, right?

Yes, good catch. Would you like me to send a separate patch?

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-25 Thread Greg KH 
On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> From: Konstantin Khlebnikov 
> 
> commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> 
> Commit:
> 
>   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> 
> .. moved sched_online_group() from css_online() to css_alloc().
> It exposes half-baked task group into global lists before initializing
> generic cgroup stuff.
> 
> LTP testcase (third in cgroup_regression_test) written for testing
> similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> 
>   BUG: unable to handle kernel NULL pointer dereference at 0008
>   IP: kernfs_path_from_node_locked+0x260/0x320
>   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
>   Call Trace:
>   ? kernfs_path_from_node+0x4f/0x60
>   kernfs_path_from_node+0x3e/0x60
>   print_rt_rq+0x44/0x2b0
>   print_rt_stats+0x7a/0xd0
>   print_cpu+0x2fc/0xe80
>   ? __might_sleep+0x4a/0x80
>   sched_debug_show+0x17/0x30
>   seq_read+0xf2/0x3b0
>   proc_reg_read+0x42/0x70
>   __vfs_read+0x28/0x130
>   ? security_file_permission+0x9b/0xc0
>   ? rw_verify_area+0x4e/0xb0
>   vfs_read+0xa5/0x170
>   SyS_read+0x46/0xa0
>   entry_SYSCALL_64_fastpath+0x1e/0xad
> 
> Here the task group is already linked into the global RCU-protected 
> 'task_groups'
> list, but the css->cgroup pointer is still NULL.
> 
> This patch reverts this chunk and moves online back to css_online().
> 
> Signed-off-by: Konstantin Khlebnikov 
> Signed-off-by: Peter Zijlstra (Intel) 
> Cc: Linus Torvalds 
> Cc: Peter Zijlstra 
> Cc: Tejun Heo 
> Cc: Thomas Gleixner 
> Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> Link: 
> http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> Signed-off-by: Ingo Molnar 
> Signed-off-by: Matt Fleming 
> ---
>  kernel/sched/core.c | 14 --
>  1 file changed, 12 insertions(+), 2 deletions(-)

What about 4.9-stable, this should go there too, right?

thanks,

greg k-h

Re: [PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-25 Thread Greg KH 
On Thu, Jul 20, 2017 at 02:53:09PM +0100, Matt Fleming wrote:
> From: Konstantin Khlebnikov 
> 
> commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.
> 
> Commit:
> 
>   2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> 
> .. moved sched_online_group() from css_online() to css_alloc().
> It exposes half-baked task group into global lists before initializing
> generic cgroup stuff.
> 
> LTP testcase (third in cgroup_regression_test) written for testing
> similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
> 
>   BUG: unable to handle kernel NULL pointer dereference at 0008
>   IP: kernfs_path_from_node_locked+0x260/0x320
>   CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
>   Call Trace:
>   ? kernfs_path_from_node+0x4f/0x60
>   kernfs_path_from_node+0x3e/0x60
>   print_rt_rq+0x44/0x2b0
>   print_rt_stats+0x7a/0xd0
>   print_cpu+0x2fc/0xe80
>   ? __might_sleep+0x4a/0x80
>   sched_debug_show+0x17/0x30
>   seq_read+0xf2/0x3b0
>   proc_reg_read+0x42/0x70
>   __vfs_read+0x28/0x130
>   ? security_file_permission+0x9b/0xc0
>   ? rw_verify_area+0x4e/0xb0
>   vfs_read+0xa5/0x170
>   SyS_read+0x46/0xa0
>   entry_SYSCALL_64_fastpath+0x1e/0xad
> 
> Here the task group is already linked into the global RCU-protected 
> 'task_groups'
> list, but the css->cgroup pointer is still NULL.
> 
> This patch reverts this chunk and moves online back to css_online().
> 
> Signed-off-by: Konstantin Khlebnikov 
> Signed-off-by: Peter Zijlstra (Intel) 
> Cc: Linus Torvalds 
> Cc: Peter Zijlstra 
> Cc: Tejun Heo 
> Cc: Thomas Gleixner 
> Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
> Link: 
> http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
> Signed-off-by: Ingo Molnar 
> Signed-off-by: Matt Fleming 
> ---
>  kernel/sched/core.c | 14 --
>  1 file changed, 12 insertions(+), 2 deletions(-)

What about 4.9-stable, this should go there too, right?

thanks,

greg k-h

[PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-20 Thread Matt Fleming 
From: Konstantin Khlebnikov 

commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.

Commit:

  2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")

.. moved sched_online_group() from css_online() to css_alloc().
It exposes half-baked task group into global lists before initializing
generic cgroup stuff.

LTP testcase (third in cgroup_regression_test) written for testing
similar race in kernels 2.6.26-2.6.28 easily triggers this oops:

  BUG: unable to handle kernel NULL pointer dereference at 0008
  IP: kernfs_path_from_node_locked+0x260/0x320
  CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
  Call Trace:
  ? kernfs_path_from_node+0x4f/0x60
  kernfs_path_from_node+0x3e/0x60
  print_rt_rq+0x44/0x2b0
  print_rt_stats+0x7a/0xd0
  print_cpu+0x2fc/0xe80
  ? __might_sleep+0x4a/0x80
  sched_debug_show+0x17/0x30
  seq_read+0xf2/0x3b0
  proc_reg_read+0x42/0x70
  __vfs_read+0x28/0x130
  ? security_file_permission+0x9b/0xc0
  ? rw_verify_area+0x4e/0xb0
  vfs_read+0xa5/0x170
  SyS_read+0x46/0xa0
  entry_SYSCALL_64_fastpath+0x1e/0xad

Here the task group is already linked into the global RCU-protected 
'task_groups'
list, but the css->cgroup pointer is still NULL.

This patch reverts this chunk and moves online back to css_online().

Signed-off-by: Konstantin Khlebnikov 
Signed-off-by: Peter Zijlstra (Intel) 
Cc: Linus Torvalds 
Cc: Peter Zijlstra 
Cc: Tejun Heo 
Cc: Thomas Gleixner 
Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
Link: 
http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
Signed-off-by: Ingo Molnar 
Signed-off-by: Matt Fleming 
---
 kernel/sched/core.c | 14 --
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20253dbc8610..7a5060bf3bbc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8237,11 +8237,20 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state 
*parent_css)
if (IS_ERR(tg))
return ERR_PTR(-ENOMEM);
 
-   sched_online_group(tg, parent);
-
return >css;
 }
 
+/* Expose task group only after completing cgroup initialization */
+static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+{
+   struct task_group *tg = css_tg(css);
+   struct task_group *parent = css_tg(css->parent);
+
+   if (parent)
+   sched_online_group(tg, parent);
+   return 0;
+}
+
 static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
 {
struct task_group *tg = css_tg(css);
@@ -8616,6 +8625,7 @@ static struct cftype cpu_files[] = {
 
 struct cgroup_subsys cpu_cgrp_subsys = {
.css_alloc  = cpu_cgroup_css_alloc,
+   .css_online = cpu_cgroup_css_online,
.css_released   = cpu_cgroup_css_released,
.css_free   = cpu_cgroup_css_free,
.fork   = cpu_cgroup_fork,
-- 
2.12.2

[PATCH v4.4.y] sched/cgroup: Move sched_online_group() back into css_online() to fix crash

2017-07-20 Thread Matt Fleming 
From: Konstantin Khlebnikov 

commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.

Commit:

  2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")

.. moved sched_online_group() from css_online() to css_alloc().
It exposes half-baked task group into global lists before initializing
generic cgroup stuff.

LTP testcase (third in cgroup_regression_test) written for testing
similar race in kernels 2.6.26-2.6.28 easily triggers this oops:

  BUG: unable to handle kernel NULL pointer dereference at 0008
  IP: kernfs_path_from_node_locked+0x260/0x320
  CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
  Call Trace:
  ? kernfs_path_from_node+0x4f/0x60
  kernfs_path_from_node+0x3e/0x60
  print_rt_rq+0x44/0x2b0
  print_rt_stats+0x7a/0xd0
  print_cpu+0x2fc/0xe80
  ? __might_sleep+0x4a/0x80
  sched_debug_show+0x17/0x30
  seq_read+0xf2/0x3b0
  proc_reg_read+0x42/0x70
  __vfs_read+0x28/0x130
  ? security_file_permission+0x9b/0xc0
  ? rw_verify_area+0x4e/0xb0
  vfs_read+0xa5/0x170
  SyS_read+0x46/0xa0
  entry_SYSCALL_64_fastpath+0x1e/0xad

Here the task group is already linked into the global RCU-protected 
'task_groups'
list, but the css->cgroup pointer is still NULL.

This patch reverts this chunk and moves online back to css_online().

Signed-off-by: Konstantin Khlebnikov 
Signed-off-by: Peter Zijlstra (Intel) 
Cc: Linus Torvalds 
Cc: Peter Zijlstra 
Cc: Tejun Heo 
Cc: Thomas Gleixner 
Fixes: 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")
Link: 
http://lkml.kernel.org/r/148655324740.424917.5302984537258726349.stgit@buzz
Signed-off-by: Ingo Molnar 
Signed-off-by: Matt Fleming 
---
 kernel/sched/core.c | 14 --
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20253dbc8610..7a5060bf3bbc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8237,11 +8237,20 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state 
*parent_css)
if (IS_ERR(tg))
return ERR_PTR(-ENOMEM);
 
-   sched_online_group(tg, parent);
-
return >css;
 }
 
+/* Expose task group only after completing cgroup initialization */
+static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+{
+   struct task_group *tg = css_tg(css);
+   struct task_group *parent = css_tg(css->parent);
+
+   if (parent)
+   sched_online_group(tg, parent);
+   return 0;
+}
+
 static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
 {
struct task_group *tg = css_tg(css);
@@ -8616,6 +8625,7 @@ static struct cftype cpu_files[] = {
 
 struct cgroup_subsys cpu_cgrp_subsys = {
.css_alloc  = cpu_cgroup_css_alloc,
+   .css_online = cpu_cgroup_css_online,
.css_released   = cpu_cgroup_css_released,
.css_free   = cpu_cgroup_css_free,
.fork   = cpu_cgroup_fork,
-- 
2.12.2