Re: [PATCH] cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated

[kernel test robot]

Hi Tejun,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tj-cgroup/for-next]
[also build test WARNING on linus/master next-20260430]
[cannot apply to v7.1-rc1]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Tejun-Heo/cgroup-Defer-css-percpu_ref-kill-on-rmdir-until-cgroup-is-depopulated/20260503-165802
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
patch link:    https://lore.kernel.org/r/20260501022943.3714461-1-tj%40kernel.org
patch subject: [PATCH] cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated
config: powerpc-randconfig-r071-20260504 (https://download.01.org/0day-ci/archive/20260504/202605040408.yt7xcKug-lkp@xxxxxxxxx/config)
compiler: powerpc-linux-gcc (GCC) 8.5.0
smatch: v0.5.0-9065-ge9cc34fd
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260504/202605040408.yt7xcKug-lkp@xxxxxxxxx/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605040408.yt7xcKug-lkp@xxxxxxxxx/

All warnings (new ones prefixed by >>):

   kernel/cgroup/cgroup.c: In function 'cgroup_apply_control_disable':
   kernel/cgroup/cgroup.c:3391:5: error: implicit declaration of function 'kill_css_sync'; did you mean 'kill_fasync'? [-Werror=implicit-function-declaration]
        kill_css_sync(css);
        ^~~~~~~~~~~~~
        kill_fasync
   kernel/cgroup/cgroup.c:3392:5: error: implicit declaration of function 'kill_css_finish'; did you mean 'kill_cad_pid'? [-Werror=implicit-function-declaration]
        kill_css_finish(css);
        ^~~~~~~~~~~~~~~
        kill_cad_pid
   kernel/cgroup/cgroup.c: At top level:
>> kernel/cgroup/cgroup.c:6047:13: warning: conflicting types for 'kill_css_sync'
    static void kill_css_sync(struct cgroup_subsys_state *css)
                ^~~~~~~~~~~~~
   kernel/cgroup/cgroup.c:6047:13: error: static declaration of 'kill_css_sync' follows non-static declaration
   kernel/cgroup/cgroup.c:3391:5: note: previous implicit declaration of 'kill_css_sync' was here
        kill_css_sync(css);
        ^~~~~~~~~~~~~
>> kernel/cgroup/cgroup.c:6087:13: warning: conflicting types for 'kill_css_finish'
    static void kill_css_finish(struct cgroup_subsys_state *css)
                ^~~~~~~~~~~~~~~
   kernel/cgroup/cgroup.c:6087:13: error: static declaration of 'kill_css_finish' follows non-static declaration
   kernel/cgroup/cgroup.c:3392:5: note: previous implicit declaration of 'kill_css_finish' was here
        kill_css_finish(css);
        ^~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors


vim +/kill_css_sync +6047 kernel/cgroup/cgroup.c

  6040	
  6041	/**
  6042	 * kill_css_sync - synchronous half of css teardown
  6043	 * @css: css being killed
  6044	 *
  6045	 * See cgroup_destroy_locked().
  6046	 */
> 6047	static void kill_css_sync(struct cgroup_subsys_state *css)
  6048	{
  6049		struct cgroup_subsys *ss = css->ss;
  6050	
  6051		lockdep_assert_held(&cgroup_mutex);
  6052	
  6053		if (css->flags & CSS_DYING)
  6054			return;
  6055	
  6056		/*
  6057		 * Call css_killed(), if defined, before setting the CSS_DYING flag
  6058		 */
  6059		if (css->ss->css_killed)
  6060			css->ss->css_killed(css);
  6061	
  6062		css->flags |= CSS_DYING;
  6063	
  6064		/*
  6065		 * This must happen before css is disassociated with its cgroup.
  6066		 * See seq_css() for details.
  6067		 */
  6068		css_clear_dir(css);
  6069	
  6070		css->cgroup->nr_dying_subsys[ss->id]++;
  6071		/*
  6072		 * Parent css and cgroup cannot be freed until after the freeing
  6073		 * of child css, see css_free_rwork_fn().
  6074		 */
  6075		while ((css = css->parent)) {
  6076			css->nr_descendants--;
  6077			css->cgroup->nr_dying_subsys[ss->id]++;
  6078		}
  6079	}
  6080	
  6081	/**
  6082	 * kill_css_finish - deferred half of css teardown
  6083	 * @css: css being killed
  6084	 *
  6085	 * See cgroup_destroy_locked().
  6086	 */
> 6087	static void kill_css_finish(struct cgroup_subsys_state *css)
  6088	{
  6089		lockdep_assert_held(&cgroup_mutex);
  6090	
  6091		/*
  6092		 * Skip on re-entry: cgroup_apply_control_disable() may have killed @css
  6093		 * earlier. cgroup_destroy_locked() can still walk it because
  6094		 * offline_css() (which NULLs cgrp->subsys[ssid]) runs async.
  6095		 */
  6096		if (percpu_ref_is_dying(&css->refcnt))
  6097			return;
  6098	
  6099		/*
  6100		 * Killing would put the base ref, but we need to keep it alive until
  6101		 * after ->css_offline().
  6102		 */
  6103		css_get(css);
  6104	
  6105		/*
  6106		 * cgroup core guarantees that, by the time ->css_offline() is invoked,
  6107		 * no new css reference will be given out via css_tryget_online(). We
  6108		 * can't simply call percpu_ref_kill() and proceed to offlining css's
  6109		 * because percpu_ref_kill() doesn't guarantee that the ref is seen as
  6110		 * killed on all CPUs on return.
  6111		 *
  6112		 * Use percpu_ref_kill_and_confirm() to get notifications as each css is
  6113		 * confirmed to be seen as killed on all CPUs.
  6114		 */
  6115		percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
  6116	}
  6117	

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