Re: [RFC][PATCH] printk: Fixup the nmi printk mess

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 10, 2015 at 04:31:55PM +0200, Petr Mladek wrote:
> > +static void __printk_nmi_flush(struct irq_work *work)
> > +{
> > +	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
> > +	int len, last_i = 0, i = 0;
> > +
> > +again:
> > +	len = seq_buf_used(&s->seq);
> > +	if (!len)
> > +		return;
> > +
> > +	/* Print line by line. */
> > +	for (; i < len; i++) {
> > +		if (s->buffer[i] == '\n') {
> > +			print_seq_line(s, last_i, i);
> > +			last_i = i + 1;
> > +		}
> > +	}
> > +	/* Check if there was a partial line. */
> > +	if (last_i < len) {
> > +		print_seq_line(s, last_i, len - 1);
> > +		pr_cont("\n");
> > +	}
> > +
> > +	/*
> > +	 * Another NMI could have come in while we were printing
> > +	 * check if nothing has been added to the buffer.
> > +	 */
> > +	if (cmpxchg_local(&s->seq.len, len, 0) != len)
> > +		goto again;
> 
> If another NMI comes at this point, it will start filling the buffer
> from the beginning. If it is fast enough, it might override the text
> that we print above.

How so? If the cmpxchg succeeded and len == 0, we flushed everything and
are done with it, if another NMI comes in and 'overwrites' it, that's
fine, right?

> > +static int vprintk_nmi(const char *fmt, va_list args)
> > +{
> > +	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
> > +	unsigned int len = seq_buf_used(&s->seq);
> > +
> > +	irq_work_queue(&s->work);
> > +	seq_buf_vprintf(&s->seq, fmt, args);
> 
> This is more critical. seq_buf_vprintf() has the following code:
> 
> 	len = vsnprintf(s->buffer + s->len, s->size - s->len, fmt, args);
> 
> The race might look like:
> 
> CPU0					CPU1
> 

No, everything is strictly per cpu.

> To be honest, I am not familiar with cmpxchg_local(). But I think that
> it can't do much difference. The value has to be synced to the other
> CPUs one day.

Nope..