Re: [RFC PATCH] nmi,printk: fix ABBA deadlock between nmi_backtrace and dump_stack_lvl

[Rik van Riel <riel@surriel.com>]

On Thu, 2024-07-18 at 09:31 +0206, John Ogness wrote:
> On 2024-07-17, Rik van Riel <riel@surriel.com> wrote:
> > I think that would do the trick. The nmi_backtrace() printk is
> > already
> > deferred, because of the check for in_nmi() in vprintk(), and this
> > change would put all the other users of
> > printk_cpu_sync_get_irqsave()
> > on the exact same footing as nmi_backtrace().
> > 
> > Combing through the code a little, it looks like that would remove
> > the potential for this deadlock to happen again.
> 
> Let's see what Petr has to say. (He'll be back on Monday.) He might
> prefer a solution that does not result in deferring printing for all
> cases. i.e. allow the console_lock if it is available, but avoid the
> spinning if it is not. Below is a patch that would achieve this.
> 
> John
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index dddb15f48d59..36f40db0bf93 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1060,6 +1060,8 @@ static int __init log_buf_len_setup(char *str)
>  early_param("log_buf_len", log_buf_len_setup);
>  
>  #ifdef CONFIG_SMP
> +static bool vprintk_emit_may_spin(void);
> +
>  #define __LOG_CPU_MAX_BUF_LEN (1 << CONFIG_LOG_CPU_MAX_BUF_SHIFT)
>  
>  static void __init log_buf_add_cpu(void)
> @@ -1090,6 +1092,7 @@ static void __init log_buf_add_cpu(void)
>  }
>  #else /* !CONFIG_SMP */
>  static inline void log_buf_add_cpu(void) {}
> +static inline bool vprintk_emit_may_spin(void) { return true };
>  #endif /* CONFIG_SMP */
>  
>  static void __init set_percpu_data_ready(void)
> @@ -2330,6 +2333,8 @@ asmlinkage int vprintk_emit(int facility, int
> level,
>  
>  	/* If called from the scheduler, we can not call up(). */
>  	if (!in_sched) {
> +		int ret;
> +
>  		/*
>  		 * The caller may be holding system-critical or
>  		 * timing-sensitive locks. Disable preemption during
> @@ -2344,7 +2349,11 @@ asmlinkage int vprintk_emit(int facility, int
> level,
>  		 * spinning variant, this context tries to take over
> the
>  		 * printing from another printing context.
>  		 */
> -		if (console_trylock_spinning())
> +		if (vprintk_emit_may_spin())
> +			ret = console_trylock_spinning();
> +		else
> +			ret = console_trylock();
> +		if (ret)
>  			console_unlock();
>  		preempt_enable();
>  	}
> @@ -4321,6 +4330,15 @@ void console_replay_all(void)
>  static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
>  static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);
>  
> +/*
> + * As documented in printk_cpu_sync_get_irqsave(), a context holding
> the
> + * printk_cpu_sync must not spin waiting for another CPU.
> + */
> +static bool vprintk_emit_may_spin(void)
> +{
> +	return (atomic_read(&printk_cpu_sync_owner) !=
> smp_processor_id());
> +}

I think the above would still deadlock, because the reported
deadlock is an ABBA deadlock between two different CPUs.

I think what the code would have to do is only trylock, and never
spin after taking the printk_cpu_sync_get_irqsave lock.

Were you thinking of moving the  this_cpu_read(printk_context)
check from vprintk() into  vprintk_emit() and use that to decide
whether to spin for the lock, or to give up if the trylock fails?

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