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

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

On Wed, 2024-07-17 at 09:22 +0206, John Ogness wrote:
> 
> The purpose of printk_cpu_sync_get_irqsave() is to avoid having the
> different backtraces being interleaved in the _ringbuffer_. It really
> isn't necessary that they are printed in that context. And indeed,
> there
> is no guarantee that they will be printed in that context anyway.
> 
> Perhaps a simple solution would be for printk_cpu_sync_get_irqsave()
> to
> call printk_deferred_enter/_exit. Something like the below patch.
> 

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.
> 
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 65c5184470f1..1a6f5aac28bf 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -315,8 +315,10 @@ extern void __printk_cpu_sync_put(void);
>  #define printk_cpu_sync_get_irqsave(flags)		\
>  	for (;;) {					\
>  		local_irq_save(flags);			\
> +		printk_deferred_enter();		\
>  		if (__printk_cpu_sync_try_get())	\
>  			break;				\
> +		printk_deferred_exit();			\
>  		local_irq_restore(flags);		\
>  		__printk_cpu_sync_wait();		\
>  	}
> @@ -329,6 +331,7 @@ extern void __printk_cpu_sync_put(void);
>  #define printk_cpu_sync_put_irqrestore(flags)	\
>  	do {					\
>  		__printk_cpu_sync_put();	\
> +		printk_deferred_exit();		\
>  		local_irq_restore(flags);	\
>  	} while (0)
>  
> 

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