Re: [PATCH v2 1/1] cgroup: replace global percpu_rwsem with signal_struct->group_rwsem when writing cgroup.procs/threads

[kernel test robot <lkp@intel.com>]

Hi Yi,

kernel test robot noticed the following build errors:

[auto build test ERROR on tj-cgroup/for-next]
[also build test ERROR on kees/for-next/execve akpm-mm/mm-everything tip/sched/core linus/master v6.17-rc4]
[cannot apply to next-20250905]
[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/Yi-Tao/cgroup-replace-global-percpu_rwsem-with-signal_struct-group_rwsem-when-writing-cgroup-procs-threads/20250904-194505
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
patch link:    https://lore.kernel.org/r/068d58f1f497bc4971c6ac0bae58bf53b98451fd.1756985260.git.escape%40linux.alibaba.com
patch subject: [PATCH v2 1/1] cgroup: replace global percpu_rwsem with signal_struct->group_rwsem when writing cgroup.procs/threads
config: i386-buildonly-randconfig-004-20250905 (https://download.01.org/0day-ci/archive/20250905/202509052150.4GQ04PJn-lkp@intel.com/config)
compiler: gcc-13 (Debian 13.3.0-16) 13.3.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250905/202509052150.4GQ04PJn-lkp@intel.com/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@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202509052150.4GQ04PJn-lkp@intel.com/

All errors (new ones prefixed by >>):

   kernel/cgroup/cgroup.c: In function 'cgroup_attach_lock':
>> kernel/cgroup/cgroup.c:2511:28: error: 'favor_dynmods' undeclared (first use in this function); did you mean 'Opt_favordynmods'?
    2511 |                 if (tsk && favor_dynmods)
         |                            ^~~~~~~~~~~~~
         |                            Opt_favordynmods
   kernel/cgroup/cgroup.c:2511:28: note: each undeclared identifier is reported only once for each function it appears in
   kernel/cgroup/cgroup.c: In function 'cgroup_attach_unlock':
   kernel/cgroup/cgroup.c:2526:28: error: 'favor_dynmods' undeclared (first use in this function); did you mean 'Opt_favordynmods'?
    2526 |                 if (tsk && favor_dynmods)
         |                            ^~~~~~~~~~~~~
         |                            Opt_favordynmods


vim +2511 kernel/cgroup/cgroup.c

  2482	
  2483	/**
  2484	 * cgroup_attach_lock - Lock for ->attach()
  2485	 * @tsk: thread group to lock
  2486	 * @lock_threadgroup: whether to down_write rwsem
  2487	 *
  2488	 * cgroup migration sometimes needs to stabilize threadgroups against forks and
  2489	 * exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
  2490	 * implementations (e.g. cpuset), also need to disable CPU hotplug.
  2491	 * Unfortunately, letting ->attach() operations acquire cpus_read_lock() can
  2492	 * lead to deadlocks.
  2493	 *
  2494	 * Bringing up a CPU may involve creating and destroying tasks which requires
  2495	 * read-locking threadgroup_rwsem, so threadgroup_rwsem nests inside
  2496	 * cpus_read_lock(). If we call an ->attach() which acquires the cpus lock while
  2497	 * write-locking threadgroup_rwsem, the locking order is reversed and we end up
  2498	 * waiting for an on-going CPU hotplug operation which in turn is waiting for
  2499	 * the threadgroup_rwsem to be released to create new tasks. For more details:
  2500	 *
  2501	 *   http://lkml.kernel.org/r/20220711174629.uehfmqegcwn2lqzu@wubuntu
  2502	 *
  2503	 * Resolve the situation by always acquiring cpus_read_lock() before optionally
  2504	 * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
  2505	 * CPU hotplug is disabled on entry.
  2506	 */
  2507	void cgroup_attach_lock(struct task_struct *tsk, bool lock_threadgroup)
  2508	{
  2509		cpus_read_lock();
  2510		if (lock_threadgroup) {
> 2511			if (tsk && favor_dynmods)
  2512				down_write(&tsk->signal->group_rwsem);
  2513			else
  2514				percpu_down_write(&cgroup_threadgroup_rwsem);
  2515		}
  2516	}
  2517	

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