Re: [PATCH] printk: fold consecutive duplicate messages

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

Hi 林濬哲,

kernel test robot noticed the following build warnings:

[auto build test WARNING on linus/master]
[also build test WARNING on v7.3-rc4 next-20260922]
[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/printk-fold-consecutive-duplicate-messages/20260921-130304
base:   linus/master
patch link:    https://lore.kernel.org/r/20260921050304.73440-1-m18667909625%40163.com
patch subject: [PATCH] printk: fold consecutive duplicate messages
config: hexagon-allnoconfig (https://download.01.org/0day-ci/archive/20260924/202609240518.RU2loaID-lkp@intel.com/config)
compiler: clang version 24.0.0git (https://github.com/llvm/llvm-project 6e714c8d91116794cb699cdf80c26afe9cda3ef3)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260924/202609240518.RU2loaID-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/202609240518.RU2loaID-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> kernel/printk/printk.c:158:7: warning: variable 'n' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
     158 |                 if (dedup_repeat >= 10 ||
         |                     ^~~~~~~~~~~~~~~~~~~~~
     159 |                     now - dedup_window_start > PRINTK_DEDUP_WINDOW_NS) {
         |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   kernel/printk/printk.c:187:6: note: uninitialized use occurs here
     187 |         if (n)
         |             ^
   kernel/printk/printk.c:158:3: note: remove the 'if' if its condition is always true
     158 |                 if (dedup_repeat >= 10 ||
         |                 ^~~~~~~~~~~~~~~~~~~~~~~~~
     159 |                     now - dedup_window_start > PRINTK_DEDUP_WINDOW_NS) {
         |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     160 |                         n = dedup_repeat;
     161 |                         dedup_repeat = 0;
     162 |                         dedup_window_start = now;
     163 |                         drop = false;
     164 |                 } else {
         |                   ~~~~~~
     165 |                         drop = true;
         |                         ~~~~~~~~~~~~
     166 |                 }
         |                 ~
   kernel/printk/printk.c:140:7: note: initialize the variable 'n' to silence this warning
     140 |         u32 n;
         |              ^
         |               = 0
   1 warning generated.


vim +158 kernel/printk/printk.c

   124	
   125	/*
   126	 * Detect consecutive duplicate printk messages and fold them away.
   127	 * Returns true if this message should be dropped.
   128	 *
   129	 * The dedup key is built from facility, level and the format string.
   130	 * Format parameters are intentionally not part of the key: va_list
   131	 * can only be consumed once, and hashing rendered text would put
   132	 * string comparisons on the printk fast path.
   133	 */
   134	static bool outputs_dedupe(int facility, int level, const char *fmt)
   135	{
   136		unsigned long flags;
   137		bool drop;
   138		u64 now;
   139		u32 key;
   140		u32 n;
   141	
   142		/* error and above must never be folded */
   143		if (level <= LOGLEVEL_ERR || !printk_dedup)
   144			return false;
   145	
   146		key = hash_64((unsigned long)(fmt ? : ""), 32) ^
   147		      hash_32(facility ^ level, 32);
   148		now = ktime_get_ns();
   149	
   150		raw_spin_lock_irqsave(&dedup_lock, flags);
   151	
   152		if (dedup_active && key == dedup_last_key) {
   153			dedup_repeat++;
   154			/*
   155			 * Force a flush when 10 repeats have accumulated or
   156			 * the 1s window has elapsed, so the summary shows up.
   157			 */
 > 158			if (dedup_repeat >= 10 ||
   159			    now - dedup_window_start > PRINTK_DEDUP_WINDOW_NS) {
   160				n = dedup_repeat;
   161				dedup_repeat = 0;
   162				dedup_window_start = now;
   163				drop = false;
   164			} else {
   165				drop = true;
   166			}
   167		} else {
   168			if (dedup_active && dedup_repeat) {
   169				n = dedup_repeat;
   170				drop = false;
   171			} else {
   172				n = 0;
   173				drop = false;
   174			}
   175			dedup_last_key = key;
   176			dedup_repeat = 0;
   177			dedup_window_start = now;
   178			dedup_active = true;
   179		}
   180	
   181		raw_spin_unlock_irqrestore(&dedup_lock, flags);
   182	
   183		/*
   184		 * Print only after dropping the lock: pr_info() re-enters
   185		 * vprintk_emit() and would deadlock on dedup_lock otherwise.
   186		 */
   187		if (n)
   188			pr_info("last message repeated %u times\n", n);
   189	
   190		return drop;
   191	}
   192	

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