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Date:   Mon, 25 May 2020 19:02:57 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Andrii Nakryiko <andriin@fb.com>,
        Alan Stern <stern@rowland.harvard.edu>, parri.andrea@gmail.com,
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        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 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.

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.