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The Calltrace is too long. I put it in the end. >>>> The issue can also be reproduced in the latest kernel. >>>> >>>> The issue is as follows. CPU3 is waiting for a spin_lock, which is got by CPU1. >>>> But CPU1 stuck in the following dead loop. >>>> >>>> irq_exit() >>>> __irq_exit_rcu() >>>> /* in_hardirq() returns false after this */ >>>> preempt_count_sub(HARDIRQ_OFFSET) >>>> tick_irq_exit() >>>> tick_nohz_irq_exit() >>>> tick_nohz_stop_sched_tick() >>>> trace_tick_stop() /* a bpf prog is hooked on this trace point */ >>>> __bpf_trace_tick_stop() >>>> bpf_trace_run2() >>>> rcu_read_unlock_special() >>>> /* will send a IPI to itself */ >>>> irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); >>>> >>>> /* after interrupt is enabled again, the irq_work is called */ >>>> asm_sysvec_irq_work() >>>> sysvec_irq_work() >>>> irq_exit() /* after handled the irq_work, we again enter into irq_exit() */ >>>> __irq_exit_rcu() >>>> ...skip... >>>> /* we queue a irq_work again, and enter a dead loop */ >>>> irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); >>> >>> This seems legitimate, Boqun and I were just talking about it. He may >>> share more thoughts but here are a few: >>> >>> Maybe we can delay subsequent clearing of the flag in >>> rcu_preempt_deferred_qs_handler() using a timer and an exponential >>> back-off? That way we are not sending too many self-IPIs. >>> >>> And reset the process at the end of a grace period. >>> >>> Or just don't send subsequent self-IPIs if we just sent one for the >>> rdp. Chances are, if we did not get the scheduler's attention during >>> the first one, we may not in subsequent ones I think. Plus we do send >>> other IPIs already if the grace period was over extended (from the FQS >>> loop), maybe we can tweak that? >> >> Thanks a lot for your reply. I think it's hard for me to fix this issue as >> above without introducing new bugs. I barely understand the RCU code. But I'm >> very glad to help test if you have any code modifiction need to. I have >> the VM and the syskaller benchmark which can reproduce the problem. > > Sure, I understand. This is already incredibly valuable so thank you again. > Will request for your testing help soon. I also have a test module now which > can sort-off reproduce this. Keep you posted! Oh sorry I meant to ask - could you provide the full kernel log and also is there a standalone reproducer syzcaller binary one can run to reproduce it in a VM? Thanks a lot.