tree: https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git for-5.15/block head: 99d26de2f6d79badc80f55b54bd90d4cb9d1ad90 commit: 5ed964f8e54eb3191b8b7b45aeb52672a0c995dc [48/54] mm: hide laptop_mode_wb_timer entirely behind the BDI API config: hexagon-randconfig-r032-20210810 (attached as .config) compiler: clang version 12.0.0 reproduce (this is a W=1 build): wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross chmod +x ~/bin/make.cross # https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/com... git remote add block https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git git fetch --no-tags block for-5.15/block git checkout 5ed964f8e54eb3191b8b7b45aeb52672a0c995dc # save the attached .config to linux build tree mkdir build_dir COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross O=build_dir ARCH=hexagon SHELL=/bin/bash If you fix the issue, kindly add following tag as appropriate Reported-by: kernel test robot <lkp(a)intel.com&gt; Note: the block/for-5.15/block HEAD 99d26de2f6d79badc80f55b54bd90d4cb9d1ad90 builds fine. It only hurts bisectibility. All errors (new ones prefixed by >>): mm/page-writeback.c:2013:6: warning: no previous prototype for function 'laptop_mode_timer_fn' [-Wmissing-prototypes] void laptop_mode_timer_fn(struct timer_list *t) ^ mm/page-writeback.c:2013:1: note: declare 'static' if the function is not intended to be used outside of this translation unit void laptop_mode_timer_fn(struct timer_list *t) ^ static mm/page-writeback.c:2026:6: warning: no previous prototype for function 'laptop_io_completion' [-Wmissing-prototypes] void laptop_io_completion(struct backing_dev_info *info) ^ mm/page-writeback.c:2026:1: note: declare 'static' if the function is not intended to be used outside of this translation unit void laptop_io_completion(struct backing_dev_info *info) ^ static void laptop_sync_completion(void) ^ include/linux/writeback.h:345:20: note: previous definition is here static inline void laptop_sync_completion(void) { } ^ 2 warnings and 1 error generated. -- timer_setup(&bdi->laptop_mode_wb_timer, laptop_mode_timer_fn, 0); ^ 1 error generated. vim +/laptop_sync_completion +2036 mm/page-writeback.c ^1da177e4c3f41 Linus Torvalds 2005-04-16 2030 ^1da177e4c3f41 Linus Torvalds 2005-04-16 2031 /* ^1da177e4c3f41 Linus Torvalds 2005-04-16 2032 * We're in laptop mode and we've just synced. The sync's writes will have ^1da177e4c3f41 Linus Torvalds 2005-04-16 2033 * caused another writeback to be scheduled by laptop_io_completion. ^1da177e4c3f41 Linus Torvalds 2005-04-16 2034 * Nothing needs to be written back anymore, so we unschedule the writeback. ^1da177e4c3f41 Linus Torvalds 2005-04-16 2035 */ ^1da177e4c3f41 Linus Torvalds 2005-04-16 @2036 void laptop_sync_completion(void) ^1da177e4c3f41 Linus Torvalds 2005-04-16 2037 { 31373d09da5b7f Matthew Garrett 2010-04-06 2038 struct backing_dev_info *bdi; 31373d09da5b7f Matthew Garrett 2010-04-06 2039 31373d09da5b7f Matthew Garrett 2010-04-06 2040 rcu_read_lock(); 31373d09da5b7f Matthew Garrett 2010-04-06 2041 31373d09da5b7f Matthew Garrett 2010-04-06 2042 list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) 31373d09da5b7f Matthew Garrett 2010-04-06 2043 del_timer(&bdi->laptop_mode_wb_timer); 31373d09da5b7f Matthew Garrett 2010-04-06 2044 31373d09da5b7f Matthew Garrett 2010-04-06 2045 rcu_read_unlock(); ^1da177e4c3f41 Linus Torvalds 2005-04-16 2046 } ^1da177e4c3f41 Linus Torvalds 2005-04-16 2047 :::::: The code at line 2036 was first introduced by commit :::::: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Linux-2.6.12-rc2 :::::: TO: Linus Torvalds <torvalds(a)ppc970.osdl.org&gt; :::::: CC: Linus Torvalds <torvalds(a)ppc970.osdl.org&gt; --- 0-DAY CI Kernel Test Service, Intel Corporation https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org