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Date:   Mon, 25 May 2020 10:45:44 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andrii Nakryiko <andriin@fb.com>,
        Alan Stern <stern@rowland.harvard.edu>, parri.andrea@gmail.com,
        will@kernel.org, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "andrii.nakryiko@gmail.com" <andrii.nakryiko@gmail.com>
Subject: Re: Some -serious- BPF-related litmus tests
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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.

s/printk()/BPF output/

One of those days...  :-/

							Thanx, Paul