Messages in this thread |  | | Date | Mon, 8 Feb 2021 22:12:20 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [External] Re: [PATCH v2] printk: fix deadlock when kernel panic |
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On (21/02/08 16:49), Muchun Song wrote: > On Mon, Feb 8, 2021 at 2:38 PM Sergey Senozhatsky > <sergey.senozhatsky@gmail.com> wrote: > > > > On (21/02/06 13:41), Muchun Song wrote: > > > We found a deadlock bug on our server when the kernel panic. It can be > > > described in the following diagram. > > > > > > CPU0: CPU1: > > > panic rcu_dump_cpu_stacks > > > kdump_nmi_shootdown_cpus nmi_trigger_cpumask_backtrace > > > register_nmi_handler(crash_nmi_callback) printk_safe_flush > > > __printk_safe_flush > > > raw_spin_lock_irqsave(&read_lock) > > > // send NMI to other processors > > > apic_send_IPI_allbutself(NMI_VECTOR) > > > // NMI interrupt, dead loop > > > crash_nmi_callback > > > > At what point does this decrement num_online_cpus()? Any chance that > > panic CPU can apic_send_IPI_allbutself() and printk_safe_flush_on_panic() > > before num_online_cpus() becomes 1? > > I took a closer look at the code. IIUC, It seems that there is no point > which decreases num_online_cpus.
So then this never re-inits the safe_read_lock?
if (num_online_cpus() > 1) return;
debug_locks_off(); raw_spin_lock_init(&safe_read_lock);
-ss
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