4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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 0000000000000008 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 <khlebnikov@xxxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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 <mingo@xxxxxxxxxx> Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/sched/core.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8253,11 +8253,20 @@ cpu_cgroup_css_alloc(struct cgroup_subsy if (IS_ERR(tg)) return ERR_PTR(-ENOMEM); - sched_online_group(tg, parent); - return &tg->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); @@ -8632,6 +8641,7 @@ 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,