On Mon 2021-02-08 23:40:07, Muchun Song wrote: > On Mon, Feb 8, 2021 at 9:12 PM Sergey Senozhatsky > <sergey.senozhat...@gmail.com> wrote: > > > > On (21/02/08 16:49), Muchun Song wrote: > > > On Mon, Feb 8, 2021 at 2:38 PM Sergey Senozhatsky > > > <sergey.senozhat...@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?
Yes, but it will also not cause the deadlock. printk_safe_flush_on_panic() will return without flushing the buffers. > Right. If we encounter this case, we do not flush printk > buffer. So, it seems my previous patch is the right fix. > Right? > > https://lore.kernel.org/patchwork/patch/1373563/ No, there is a risk of deadlock caused by logbuf_lock, see https://lore.kernel.org/lkml/YB0nggSa7a95UCIK@alley/ > > if (num_online_cpus() > 1) > > return; > > > > debug_locks_off(); > > raw_spin_lock_init(&safe_read_lock); > > > > -ss I prefer this approach. It is straightforward because it handles read_lock the same way as logbuf_lock. IMHO, it is not worth inventing any more complexity. Both logbuf_lock and read_lock are obsoleted by the lockless ringbuffer. And we need something simple to get backported to the already released kernels. Best Regards, Petr
