Re: Some -serious- BPF-related litmus tests

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, May 25, 2020 at 10:21:54AM -0700, Paul E. McKenney wrote:
> 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.

Any idea why this choice of locking-based ring buffer implementation in BPF?
The ftrace ring buffer can support NMI interruptions as well for writes.

Also, is it possible for BPF to reuse the ftrace ring buffer implementation
or does it not meet the requirements?

thanks,

 - Joel