Re: [PATCH 1/2] sched: Introduce rcuwait machinery

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

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Hi Davidlohr,

[auto build test ERROR on tip/auto-latest]
[also build test ERROR on v4.9 next-20161222]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Davidlohr-Bueso/sched-Introduce-rcuwait/20161223-020109
config: i386-randconfig-s1-201651 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

Note: the linux-review/Davidlohr-Bueso/sched-Introduce-rcuwait/20161223-020109 HEAD 9e9d238f94d5aa8e348e7e70585533fe0dbd373b builds fine.
      It only hurts bisectibility.

All error/warnings (new ones prefixed by >>):

>> kernel/exit.c:285:29: warning: 'struct rcuwait' declared inside parameter list will not be visible outside of this definition or declaration
    void rcuwait_trywake(struct rcuwait *w)
                                ^~~~~~~
   In file included from include/linux/srcu.h:33:0,
                    from include/linux/notifier.h:15,
                    from include/linux/memory_hotplug.h:6,
                    from include/linux/mmzone.h:751,
                    from include/linux/gfp.h:5,
                    from include/linux/mm.h:9,
                    from kernel/exit.c:7:
   kernel/exit.c: In function 'rcuwait_trywake':
>> kernel/exit.c:308:26: error: dereferencing pointer to incomplete type 'struct rcuwait'
     task = rcu_dereference(w->task);
                             ^
   include/linux/rcupdate.h:606:10: note: in definition of macro '__rcu_dereference_check'
     typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
             ^
   include/linux/rcupdate.h:786:28: note: in expansion of macro 'rcu_dereference_check'
    #define rcu_dereference(p) rcu_dereference_check(p, 0)
                               ^~~~~~~~~~~~~~~~~~~~~
>> kernel/exit.c:308:9: note: in expansion of macro 'rcu_dereference'
     task = rcu_dereference(w->task);
            ^~~~~~~~~~~~~~~

vim +308 kernel/exit.c

   279		if (!sighand)
   280			return NULL;
   281	
   282		return task;
   283	}
   284	
 > 285	void rcuwait_trywake(struct rcuwait *w)
   286	{
   287		struct task_struct *task;
   288	
   289		rcu_read_lock();
   290	
   291		/*
   292		 * Order condition vs @task, such that everything prior to the load
   293		 * of @task is visible. This is the condition as to why the user called
   294		 * rcuwait_trywake() in the first place. Pairs with set_current_state()
   295		 * barrier (A) in rcuwait_wait_event().
   296		 *
   297		 *    WAIT                WAKE
   298		 *    [S] tsk = current	  [S] cond = true
   299		 *        MB (A)	      MB (B)
   300		 *    [L] cond		  [L] tsk
   301		 */
   302		smp_rmb(); /* (B) */
   303	
   304		/*
   305		 * Avoid using task_rcu_dereference() magic as long as we are careful,
   306		 * see comment in rcuwait_wait_event() regarding ->exit_state.
   307		 */
 > 308		task = rcu_dereference(w->task);
   309		if (task)
   310			wake_up_process(task);
   311		rcu_read_unlock();

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