Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (01/19/18 13:20), Steven Rostedt wrote:
[..]
> I was thinking about this a bit more, and instead of offloading a
> recursive printk, perhaps its best to simply throttle it. Because the
> problem may not go away if a printk thread takes over, because the bug
> is really the printk infrastructure filling the printk buffer keeping
> printk from ever stopping.

right. I didn't quite got it how that would help. if we would
kick_offload every time we add new printks after call_console_drivers(),
then we can just end up in a kick_offload loop traveling across all CPUs.

[..]
>  asmlinkage int vprintk_emit(int facility, int level,
>  			    const char *dict, size_t dictlen,
> @@ -1849,6 +1918,17 @@ asmlinkage int vprintk_emit(int facility, int level,
>  
>  	/* This stops the holder of console_sem just where we want him */
>  	logbuf_lock_irqsave(flags);
> +
> +	if (recursion_check_test()) {
> +		/* A printk happened within a printk at the same context */
> +		if (this_cpu_inc_return(recursion_count) > recursion_max) {
> +			atomic_inc(&recursion_overflow);
> +			logbuf_unlock_irqrestore(flags);
> +			printed_len = 0;
> +			goto out;
> +		}
> +	}

didn't have time to look at this carefully, but is this possible?

printks from console_unlock()->call_console_drivers() are redirected
to printk_safe buffer. we need irq_work on that CPU to flush its
printk_safe buffer.

>  EXPORT_SYMBOL(vprintk_emit);
> @@ -2343,9 +2428,14 @@ void console_unlock(void)
>  			seen_seq = log_next_seq;
>  		}
>  
> -		if (console_seq < log_first_seq) {
> +		if (console_seq < log_first_seq || atomic_read(&recursion_overflow)) {
> +			size_t missed;
> +
> +			missed = atomic_xchg(&recursion_overflow, 0);
> +			missed += log_first_seq - console_seq;
> +
>  			len = sprintf(text, "** %u printk messages dropped **\n",
> -				      (unsigned)(log_first_seq - console_seq));
> +				      (unsigned)missed);
>  
>  			/* messages are gone, move to first one */
>  			console_seq = log_first_seq;

how are we going to distinguish between lockdep splats, for instance,
or WARNs from call_console_drivers() -> foo_write(), which are valuable,
and kmalloc() print outs, which might be less valuable? are we going to
lose all of them now? then we can do a much simpler thing - steal one
bit from `printk_context' and use if for a new PRINTK_NOOP_CONTEXT, which
will be set around call_console_drivers(). vprintk_func() would redirect
printks to vprintk_noop(fmt, args), which will do nothing.

	-ss