lkml.org 
[lkml]   [2021]   [Feb]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [External] Re: [PATCH v2] printk: fix deadlock when kernel panic
On Mon 2021-02-08 23:40:07, Muchun Song wrote:
> On Mon, Feb 8, 2021 at 9:12 PM Sergey Senozhatsky
> <sergey.senozhatsky@gmail.com> wrote:
> >
> > 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?

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

\
 
 \ /
  Last update: 2021-02-09 09:42    [W:0.037 / U:0.356 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site