tree: https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git coresched head: 680ec43dead6f0f84db19ef07fc0f5ea44e9f2b3 commit: e7a440f33a800ee9fdf07defb1176932ab4a59b1 [14/39] sched: Simplify the core pick loop for optimized case config: x86_64-randconfig-m001-20201113 (attached as .config) compiler: gcc-9 (Debian 9.3.0-15) 9.3.0 If you fix the issue, kindly add following tag as appropriate Reported-by: kernel test robot <lkp(a)intel.com&gt; Reported-by: Dan Carpenter <dan.carpenter(a)oracle.com&gt; smatch warnings: kernel/sched/core.c:4708 pick_next_task() error: we previously assumed 'next' could be null (see line 4704) vim +/next +4708 kernel/sched/core.c 620989bb8ec95d Peter Zijlstra 2020-06-30 4634 static struct task_struct * 620989bb8ec95d Peter Zijlstra 2020-06-30 4635 pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) 620989bb8ec95d Peter Zijlstra 2020-06-30 4636 { 620989bb8ec95d Peter Zijlstra 2020-06-30 4637 struct task_struct *next, *max = NULL; 620989bb8ec95d Peter Zijlstra 2020-06-30 4638 const struct sched_class *class; 620989bb8ec95d Peter Zijlstra 2020-06-30 4639 const struct cpumask *smt_mask; e4fe729c9bf5c1 Joel Fernandes (Google 2020-06-30 4640) bool fi_before = false; 620989bb8ec95d Peter Zijlstra 2020-06-30 4641 bool need_sync; 620989bb8ec95d Peter Zijlstra 2020-06-30 4642 int i, j, cpu; 620989bb8ec95d Peter Zijlstra 2020-06-30 4643 620989bb8ec95d Peter Zijlstra 2020-06-30 4644 if (!sched_core_enabled(rq)) 620989bb8ec95d Peter Zijlstra 2020-06-30 4645 return __pick_next_task(rq, prev, rf); 620989bb8ec95d Peter Zijlstra 2020-06-30 4646 620989bb8ec95d Peter Zijlstra 2020-06-30 4647 cpu = cpu_of(rq); 620989bb8ec95d Peter Zijlstra 2020-06-30 4648 620989bb8ec95d Peter Zijlstra 2020-06-30 4649 /* Stopper task is switching into idle, no need core-wide selection. */ 620989bb8ec95d Peter Zijlstra 2020-06-30 4650 if (cpu_is_offline(cpu)) { 620989bb8ec95d Peter Zijlstra 2020-06-30 4651 /* 620989bb8ec95d Peter Zijlstra 2020-06-30 4652 * Reset core_pick so that we don't enter the fastpath when 620989bb8ec95d Peter Zijlstra 2020-06-30 4653 * coming online. core_pick would already be migrated to 620989bb8ec95d Peter Zijlstra 2020-06-30 4654 * another cpu during offline. 620989bb8ec95d Peter Zijlstra 2020-06-30 4655 */ 620989bb8ec95d Peter Zijlstra 2020-06-30 4656 rq->core_pick = NULL; 620989bb8ec95d Peter Zijlstra 2020-06-30 4657 return __pick_next_task(rq, prev, rf); 620989bb8ec95d Peter Zijlstra 2020-06-30 4658 } 620989bb8ec95d Peter Zijlstra 2020-06-30 4659 620989bb8ec95d Peter Zijlstra 2020-06-30 4660 /* 620989bb8ec95d Peter Zijlstra 2020-06-30 4661 * If there were no {en,de}queues since we picked (IOW, the task 620989bb8ec95d Peter Zijlstra 2020-06-30 4662 * pointers are all still valid), and we haven't scheduled the last 620989bb8ec95d Peter Zijlstra 2020-06-30 4663 * pick yet, do so now. 620989bb8ec95d Peter Zijlstra 2020-06-30 4664 * 620989bb8ec95d Peter Zijlstra 2020-06-30 4665 * rq->core_pick can be NULL if no selection was made for a CPU because 620989bb8ec95d Peter Zijlstra 2020-06-30 4666 * it was either offline or went offline during a sibling's core-wide 620989bb8ec95d Peter Zijlstra 2020-06-30 4667 * selection. In this case, do a core-wide selection. 620989bb8ec95d Peter Zijlstra 2020-06-30 4668 */ 620989bb8ec95d Peter Zijlstra 2020-06-30 4669 if (rq->core->core_pick_seq == rq->core->core_task_seq && 620989bb8ec95d Peter Zijlstra 2020-06-30 4670 rq->core->core_pick_seq != rq->core_sched_seq && 620989bb8ec95d Peter Zijlstra 2020-06-30 4671 rq->core_pick) { 620989bb8ec95d Peter Zijlstra 2020-06-30 4672 WRITE_ONCE(rq->core_sched_seq, rq->core->core_pick_seq); 620989bb8ec95d Peter Zijlstra 2020-06-30 4673 620989bb8ec95d Peter Zijlstra 2020-06-30 4674 next = rq->core_pick; 620989bb8ec95d Peter Zijlstra 2020-06-30 4675 if (next != prev) { 620989bb8ec95d Peter Zijlstra 2020-06-30 4676 put_prev_task(rq, prev); 620989bb8ec95d Peter Zijlstra 2020-06-30 4677 set_next_task(rq, next); 620989bb8ec95d Peter Zijlstra 2020-06-30 4678 } 620989bb8ec95d Peter Zijlstra 2020-06-30 4679 620989bb8ec95d Peter Zijlstra 2020-06-30 4680 rq->core_pick = NULL; 620989bb8ec95d Peter Zijlstra 2020-06-30 4681 return next; 620989bb8ec95d Peter Zijlstra 2020-06-30 4682 } 620989bb8ec95d Peter Zijlstra 2020-06-30 4683 620989bb8ec95d Peter Zijlstra 2020-06-30 4684 put_prev_task_balance(rq, prev, rf); 620989bb8ec95d Peter Zijlstra 2020-06-30 4685 620989bb8ec95d Peter Zijlstra 2020-06-30 4686 smt_mask = cpu_smt_mask(cpu); 620989bb8ec95d Peter Zijlstra 2020-06-30 4687 need_sync = !!rq->core->core_cookie; 620989bb8ec95d Peter Zijlstra 2020-06-30 4688 620989bb8ec95d Peter Zijlstra 2020-06-30 4689 /* reset state */ 620989bb8ec95d Peter Zijlstra 2020-06-30 4690 rq->core->core_cookie = 0UL; ba9df3989cb9de Vineeth Pillai 2020-08-28 4691 if (rq->core->core_forceidle) { ba9df3989cb9de Vineeth Pillai 2020-08-28 4692 need_sync = true; e4fe729c9bf5c1 Joel Fernandes (Google 2020-06-30 4693) fi_before = true; ba9df3989cb9de Vineeth Pillai 2020-08-28 4694 rq->core->core_forceidle = false; ba9df3989cb9de Vineeth Pillai 2020-08-28 4695 } e7a440f33a800e Joel Fernandes (Google 2020-11-05 4696) e7a440f33a800e Joel Fernandes (Google 2020-11-05 4697) /* e7a440f33a800e Joel Fernandes (Google 2020-11-05 4698) * Optimize for common case where this CPU has no cookies e7a440f33a800e Joel Fernandes (Google 2020-11-05 4699) * and there are no cookied tasks running on siblings. e7a440f33a800e Joel Fernandes (Google 2020-11-05 4700) */ e7a440f33a800e Joel Fernandes (Google 2020-11-05 4701) if (!need_sync) { e7a440f33a800e Joel Fernandes (Google 2020-11-05 4702) for_each_class(class) { e7a440f33a800e Joel Fernandes (Google 2020-11-05 4703) next = class->pick_task(rq); e7a440f33a800e Joel Fernandes (Google 2020-11-05 @4704) if (next) e7a440f33a800e Joel Fernandes (Google 2020-11-05 4705) break; ^^^^^ If we exit the loop without hitting this break e7a440f33a800e Joel Fernandes (Google 2020-11-05 4706) } e7a440f33a800e Joel Fernandes (Google 2020-11-05 4707) e7a440f33a800e Joel Fernandes (Google 2020-11-05 @4708) if (!next->core_cookie) { ^^^^^^ then this will crash. e7a440f33a800e Joel Fernandes (Google 2020-11-05 4709) rq->core_pick = NULL; e7a440f33a800e Joel Fernandes (Google 2020-11-05 4710) goto done; e7a440f33a800e Joel Fernandes (Google 2020-11-05 4711) } e7a440f33a800e Joel Fernandes (Google 2020-11-05 4712) need_sync = true; e7a440f33a800e Joel Fernandes (Google 2020-11-05 4713) } e7a440f33a800e Joel Fernandes (Google 2020-11-05 4714) 620989bb8ec95d Peter Zijlstra 2020-06-30 4715 for_each_cpu(i, smt_mask) { 620989bb8ec95d Peter Zijlstra 2020-06-30 4716 struct rq *rq_i = cpu_rq(i); 620989bb8ec95d Peter Zijlstra 2020-06-30 4717 --- 0-DAY CI Kernel Test Service, Intel Corporation https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org