[rcu:dev.2018.10.03a 44/73] kernel/rcu/tree.c:2704:5: warning: 'max_cpu' may be used uninitialized in this function

[rcu:dev.2018.10.03a 44/73] kernel/rcu/tree.c:2704:5: warning: 'max_cpu' may be used uninitialized in this function

[kbuild test robot]

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tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2018.10.03a
head:   ef44a26c84188888f97e5d7b3f8e4f8c0decd248
commit: 813f47a94e3b61439bba90340b532f3a6319d4f5 [44/73] rcu: Print per-CPU callback counts for forward-progress failures
config: x86_64-randconfig-x006-10081527 (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
        git checkout 813f47a94e3b61439bba90340b532f3a6319d4f5
        # save the attached .config to linux build tree
        make ARCH=x86_64 

Note: it may well be a FALSE warning. FWIW you are at least aware of it now.
http://gcc.gnu.org/wiki/Better_Uninitialized_Warnings

All warnings (new ones prefixed by >>):

   kernel/rcu/tree.c: In function 'rcu_fwd_progress_check':
>> kernel/rcu/tree.c:2704:5: warning: 'max_cpu' may be used uninitialized in this function [-Wmaybe-uninitialized]
     if (max_cpu >= 0)
        ^
>> kernel/rcu/tree.c:2699:6: warning: 'max_cbs' may be used uninitialized in this function [-Wmaybe-uninitialized]
      if (cbs <= max_cbs)
         ^

vim +/max_cpu +2704 kernel/rcu/tree.c

  2670	
  2671	/*
  2672	 * Do a forward-progress check for rcutorture.  This is normally invoked
  2673	 * due to an OOM event.  The argument "j" gives the time period during
  2674	 * which rcutorture would like progress to have been made.
  2675	 */
  2676	void rcu_fwd_progress_check(unsigned long j)
  2677	{
  2678		unsigned long cbs;
  2679		int cpu;
  2680		unsigned long max_cbs;
  2681		int max_cpu;
  2682		struct rcu_data *rdp;
  2683	
  2684		if (rcu_gp_in_progress()) {
  2685			show_rcu_gp_kthreads();
  2686		} else {
  2687			preempt_disable();
  2688			rdp = this_cpu_ptr(&rcu_data);
  2689			rcu_check_gp_start_stall(rdp->mynode, rdp, j);
  2690			preempt_enable();
  2691		}
  2692		for_each_possible_cpu(cpu) {
  2693			cbs = rcu_get_n_cbs_cpu(cpu);
  2694			if (!cbs)
  2695				continue;
  2696			if (max_cpu < 0)
  2697				pr_info("%s: callbacks", __func__);
  2698			pr_cont(" %d: %lu", cpu, cbs);
> 2699			if (cbs <= max_cbs)
  2700				continue;
  2701			max_cbs = cbs;
  2702			max_cpu = cpu;
  2703		}
> 2704		if (max_cpu >= 0)
  2705			pr_cont("\n");
  2706	}
  2707	EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
  2708	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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Re: [rcu:dev.2018.10.03a 44/73] kernel/rcu/tree.c:2704:5: warning: 'max_cpu' may be used uninitialized in this function

[Paul E. McKenney]

On Mon, Oct 08, 2018 at 07:25:58PM +0800, kbuild test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2018.10.03a
> head:   ef44a26c84188888f97e5d7b3f8e4f8c0decd248
> commit: 813f47a94e3b61439bba90340b532f3a6319d4f5 [44/73] rcu: Print per-CPU callback counts for forward-progress failures
> config: x86_64-randconfig-x006-10081527 (attached as .config)
> compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
> reproduce:
>         git checkout 813f47a94e3b61439bba90340b532f3a6319d4f5
>         # save the attached .config to linux build tree
>         make ARCH=x86_64 
> 
> Note: it may well be a FALSE warning. FWIW you are at least aware of it now.
> http://gcc.gnu.org/wiki/Better_Uninitialized_Warnings
> 
> All warnings (new ones prefixed by >>):
> 
>    kernel/rcu/tree.c: In function 'rcu_fwd_progress_check':
> >> kernel/rcu/tree.c:2704:5: warning: 'max_cpu' may be used uninitialized in this function [-Wmaybe-uninitialized]
>      if (max_cpu >= 0)
>         ^
> >> kernel/rcu/tree.c:2699:6: warning: 'max_cbs' may be used uninitialized in this function [-Wmaybe-uninitialized]
>       if (cbs <= max_cbs)
>          ^

Good catch!  A fix is on its way.

							Thanx, Paul

> vim +/max_cpu +2704 kernel/rcu/tree.c
> 
>   2670	
>   2671	/*
>   2672	 * Do a forward-progress check for rcutorture.  This is normally invoked
>   2673	 * due to an OOM event.  The argument "j" gives the time period during
>   2674	 * which rcutorture would like progress to have been made.
>   2675	 */
>   2676	void rcu_fwd_progress_check(unsigned long j)
>   2677	{
>   2678		unsigned long cbs;
>   2679		int cpu;
>   2680		unsigned long max_cbs;
>   2681		int max_cpu;
>   2682		struct rcu_data *rdp;
>   2683	
>   2684		if (rcu_gp_in_progress()) {
>   2685			show_rcu_gp_kthreads();
>   2686		} else {
>   2687			preempt_disable();
>   2688			rdp = this_cpu_ptr(&rcu_data);
>   2689			rcu_check_gp_start_stall(rdp->mynode, rdp, j);
>   2690			preempt_enable();
>   2691		}
>   2692		for_each_possible_cpu(cpu) {
>   2693			cbs = rcu_get_n_cbs_cpu(cpu);
>   2694			if (!cbs)
>   2695				continue;
>   2696			if (max_cpu < 0)
>   2697				pr_info("%s: callbacks", __func__);
>   2698			pr_cont(" %d: %lu", cpu, cbs);
> > 2699			if (cbs <= max_cbs)
>   2700				continue;
>   2701			max_cbs = cbs;
>   2702			max_cpu = cpu;
>   2703		}
> > 2704		if (max_cpu >= 0)
>   2705			pr_cont("\n");
>   2706	}
>   2707	EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
>   2708	
> 
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation