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SubjectRe: [External] Re: [PATCH] printk: fix deadlock when kernel panic
On Fri, Feb 5, 2021 at 7:09 PM Petr Mladek <pmladek@suse.com> wrote:
>
> Hi,
>
> first, I am sorry for the late reply. I have been somehow distracted
> this week.
>
> On Mon 2021-02-01 18:26:01, 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
> > printk_safe_flush_on_panic
> > printk_safe_flush
> > __printk_safe_flush
> > // deal lock
> > raw_spin_lock_irqsave(&read_lock)
>
> Great catch!
>
> > kernel/printk/printk_safe.c | 58 ++++++++++++++++++++++++++-------------------
> > 1 file changed, 34 insertions(+), 24 deletions(-)
> >
> > diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> > index a0e6f746de6c..86d9fa74ac5c 100644
> > --- a/kernel/printk/printk_safe.c
> > +++ b/kernel/printk/printk_safe.c
> > @@ -174,30 +174,13 @@ static void report_message_lost(struct printk_safe_seq_buf *s)
> > printk_deferred("Lost %d message(s)!\n", lost);
> > }
> >
> > -/*
> > - * Flush data from the associated per-CPU buffer. The function
> > - * can be called either via IRQ work or independently.
> > - */
> > -static void __printk_safe_flush(struct irq_work *work)
> > +static void __printk_safe_flush_work(struct irq_work *work)
> > {
> > - static raw_spinlock_t read_lock =
> > - __RAW_SPIN_LOCK_INITIALIZER(read_lock);
> > struct printk_safe_seq_buf *s =
> > container_of(work, struct printk_safe_seq_buf, work);
> > - unsigned long flags;
> > size_t len;
> > - int i;
> > + int i = 0;
> >
> > - /*
> > - * The lock has two functions. First, one reader has to flush all
> > - * available message to make the lockless synchronization with
> > - * writers easier. Second, we do not want to mix messages from
> > - * different CPUs. This is especially important when printing
> > - * a backtrace.
> > - */
> > - raw_spin_lock_irqsave(&read_lock, flags);
> > -
> > - i = 0;
> > more:
> > len = atomic_read(&s->len);
> >
> > @@ -232,6 +215,26 @@ static void __printk_safe_flush(struct irq_work *work)
> >
> > out:
> > report_message_lost(s);
> > +}
> > +
> > +/*
> > + * Flush data from the associated per-CPU buffer. The function
> > + * can be called either via IRQ work or independently.
> > + */
> > +static void printk_safe_flush_work(struct irq_work *work)
> > +{
> > + unsigned long flags;
> > + static DEFINE_RAW_SPINLOCK(read_lock);
> > +
> > + /*
> > + * The lock has two functions. First, one reader has to flush all
> > + * available message to make the lockless synchronization with
> > + * writers easier. Second, we do not want to mix messages from
> > + * different CPUs. This is especially important when printing
> > + * a backtrace.
> > + */
> > + raw_spin_lock_irqsave(&read_lock, flags);
> > + __printk_safe_flush_work(work);
> > raw_spin_unlock_irqrestore(&read_lock, flags);
> > }
> >
> > @@ -248,9 +251,9 @@ void printk_safe_flush(void)
> >
> > for_each_possible_cpu(cpu) {
> > #ifdef CONFIG_PRINTK_NMI
> > - __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
> > + printk_safe_flush_work(&per_cpu(nmi_print_seq, cpu).work);
> > #endif
> > - __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
> > + printk_safe_flush_work(&per_cpu(safe_print_seq, cpu).work);
> > }
> > }
> >
> > @@ -266,6 +269,8 @@ void printk_safe_flush(void)
> > */
> > void printk_safe_flush_on_panic(void)
> > {
> > + int cpu;
> > +
> > /*
> > * Make sure that we could access the main ring buffer.
> > * Do not risk a double release when more CPUs are up.
> > @@ -278,7 +283,12 @@ void printk_safe_flush_on_panic(void)
> > raw_spin_lock_init(&logbuf_lock);
> > }
> >
> > - printk_safe_flush();
> > + for_each_possible_cpu(cpu) {
> > +#ifdef CONFIG_PRINTK_NMI
> > + __printk_safe_flush_work(&per_cpu(nmi_print_seq, cpu).work);
> > +#endif
> > + __printk_safe_flush_work(&per_cpu(safe_print_seq, cpu).work);
> > + }
>
> There might be two CPUs running this code when more CPUs are still running.
> The might definitely be the case here:
>
> if (!_crash_kexec_post_notifiers) {
> printk_safe_flush_on_panic();
> __crash_kexec(NULL);
>
> __printk_safe_flush_work() is relatively safe even in this case.
> It accesses only data that are limited by s->len that is updated
> atomically.
>
> But it calls:
>
> + printk_safe_flush_on_panic()
> + __printk_safe_flush_work()
> + printk_safe_flush_buffer()
> + printk_safe_flush_line()
> + printk_deferred()
>
> where printk_deferred() used logbuf_lock() until v5.11-rc1, see
> the commit b031a684bfd01d633c ("printk: remove logbuf_lock
> writer-protection of ringbuffer").
>
> We need a solution that might be backported back to v4.7-rc1
> where logbuf_lock() might cause a deadlock.

Agree.

>
> Also we really want to call printk_safe_flush_on_panic()
> in panic(). It is trying hard but it should not cause
> deadlock.
>
> Therefore, I suggest to handle read_lock the same way in
> printk_safe_flush_on_panic() as we handle logbuf_lock there.
>
> I mean to make it global in printk_safe.c:
>
> static DEFINE_RAW_SPINLOCK(safe_read_lock);
>
> and do something like this in printk_safe_flush_on_panic()
>
> if (raw_spin_is_locked(&safe_read_lock)) {
> if (num_online_cpus() > 1)
> return;
>
> debug_locks_off();
> raw_spin_lock_init(&safe_read_lock);
> }
>
> How does that sound, please?

Great. It is also a solution. It seems simple. I will do this.
Thanks.

>
> Best Regards,
> Petr

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