Re: Some -serious- BPF-related litmus tests

[Paul E. McKenney]

On Mon, May 25, 2020 at 07:02:57PM +0200, Peter Zijlstra wrote:
> On Mon, May 25, 2020 at 08:47:30AM -0700, Paul E. McKenney wrote:
> > On Mon, May 25, 2020 at 01:25:21PM +0200, Peter Zijlstra wrote:
> 
> > > That is; how can you use a spinlock on the producer side at all?
> > 
> > So even trylock is now forbidden in NMI handlers?  If so, why?
> 
> The litmus tests don't have trylock.

Fair point.

> But you made me look at the actual patch:
> 
> +static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
> +{
> +	unsigned long cons_pos, prod_pos, new_prod_pos, flags;
> +	u32 len, pg_off;
> +	struct bpf_ringbuf_hdr *hdr;
> +
> +	if (unlikely(size > RINGBUF_MAX_RECORD_SZ))
> +		return NULL;
> +
> +	len = round_up(size + BPF_RINGBUF_HDR_SZ, 8);
> +	cons_pos = smp_load_acquire(&rb->consumer_pos);
> +
> +	if (in_nmi()) {
> +		if (!spin_trylock_irqsave(&rb->spinlock, flags))
> +			return NULL;
> +	} else {
> +		spin_lock_irqsave(&rb->spinlock, flags);
> +	}
> 
> And that is of course utter crap. That's like saying you don't care
> about your NMI data.

Almost.  It is really saying that -if- there is sufficient lock
contention, printk()s will be lost.  Just as they always have been if
there is more printk() volume than can be accommodated.

							Thanx, Paul