Subject: Re: [PATCH linux-rcu] docs/litmus-tests: add BPF ringbuf MPSC litmus tests

From: Joel Fernandes <[email-masked]>
Date: Fri, 29 May 2020 13:34:32 -0400

Hi Andrii,

On Thu, May 28, 2020 at 10:50:30PM -0700, Andrii Nakryiko wrote:
> > [...]
> > > diff --git a/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus b/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus
> > > new file mode 100644
> > > index 000000000000..558f054fb0b4
> > > --- /dev/null
> > > +++ b/Documentation/litmus-tests/bpf-rb/bpf-rb+1p1c+bounded.litmus
> > > @@ -0,0 +1,91 @@
> > > +C bpf-rb+1p1c+bounded
> > > +
> > > +(*
> > > + * Result: Always
> > > + *
> > > + * This litmus test validates BPF ring buffer implementation under the
> > > + * following assumptions:
> > > + * - 1 producer;
> > > + * - 1 consumer;
> > > + * - ring buffer has capacity for only 1 record.
> > > + *
> > > + * Expectations:
> > > + * - 1 record pushed into ring buffer;
> > > + * - 0 or 1 element is consumed.
> > > + * - no failures.
> > > + *)
> > > +
> > > +{
> > > +     atomic_t dropped;
> > > +}
> > > +
> > > +P0(int *lenFail, int *len1, int *cx, int *px)
> > > +{
> > > +     int *rLenPtr;
> > > +     int rLen;
> > > +     int rPx;
> > > +     int rCx;
> > > +     int rFail;
> > > +
> > > +     rFail = 0;
> > > +
> > > +     rCx = smp_load_acquire(cx);
> > > +     rPx = smp_load_acquire(px);
> >
> > Is it possible for you to put some more comments around which ACQUIRE is
> > paired with which RELEASE? And, in general more comments around the reason
> > for a certain memory barrier and what pairs with what. In the kernel sources,
> > the barriers needs a comment anyway.

This was the comment earlier that was missed.

> > > +     if (rCx < rPx) {
> > > +             if (rCx == 0) {
> > > +                     rLenPtr = len1;
> > > +             } else {
> > > +                     rLenPtr = lenFail;
> > > +                     rFail = 1;
> > > +             }
> > > +
> > > +             rLen = smp_load_acquire(rLenPtr);
> > > +             if (rLen == 0) {
> > > +                     rFail = 1;
> > > +             } else if (rLen == 1) {
> > > +                     rCx = rCx + 1;
> > > +                     smp_store_release(cx, rCx);
> > > +             }
> > > +     }
> > > +}
> > > +
> > > +P1(int *lenFail, int *len1, spinlock_t *rb_lock, int *px, int *cx, atomic_t *dropped)
> > > +{
> > > +     int rPx;
> > > +     int rCx;
> > > +     int rFail;
> > > +     int *rLenPtr;
> > > +
> > > +     rFail = 0;
> > > +
> > > +     rCx = smp_load_acquire(cx);
> > > +     spin_lock(rb_lock);
> > > +
> > > +     rPx = *px;
> > > +     if (rPx - rCx >= 1) {
> > > +             atomic_inc(dropped);
> >
> > Why does 'dropped' need to be atomic if you are always incrementing under a
> > lock?
> 
> It doesn't, strictly speaking, but making it atomic in litmus test was
> just more convenient, especially that I initially also had a lock-less
> variant of this algorithm.

Ok, that's fine.

> >
> > > +             spin_unlock(rb_lock);
> > > +     } else {
> > > +             if (rPx == 0) {
> > > +                     rLenPtr = len1;
> > > +             } else {
> > > +                     rLenPtr = lenFail;
> > > +                     rFail = 1;
> > > +             }
> > > +
> > > +             *rLenPtr = -1;
> >
> > Clarify please the need to set the length intermittently to -1. Thanks.
> 
> This corresponds to setting a "busy bit" in kernel implementation.
> These litmus tests are supposed to be correlated with in-kernel
> implementation, I'm not sure I want to maintain extra 4 copies of
> comments here and in kernel code. Especially for 2-producer cases,
> there are 2 identical P1 and P2, which is unfortunate, but I haven't
> figured out how to have a re-usable pieces of code with litmus tests
> :)

I disagree that comments related to memory ordering are optional. IMHO, the
documentation should be clear from a memory ordering standpoint. After all,
good Documentation/ always clarifies something / some concept to the reader
right? :-) Please have mercy on me, I am just trying to learn *your*
Documentation ;-)

> > > diff --git a/Documentation/litmus-tests/bpf-rb/bpf-rb+2p1c+bounded.litmus b/Documentation/litmus-tests/bpf-rb/bpf-rb+2p1c+bounded.litmus
[...]
> > > +P1(int *lenFail, int *len1, spinlock_t *rb_lock, int *px, int *cx, atomic_t *dropped)
> > > +{
> > > +     int rPx;
> > > +     int rCx;
> > > +     int rFail;
> > > +     int *rLenPtr;
> > > +
> > > +     rFail = 0;
> > > +     rLenPtr = lenFail;
> > > +
> > > +     rCx = smp_load_acquire(cx);
> > > +     spin_lock(rb_lock);
> > > +
> > > +     rPx = *px;
> > > +     if (rPx - rCx >= 1) {
> > > +             atomic_inc(dropped);
> > > +             spin_unlock(rb_lock);
> > > +     } else {
> > > +             if (rPx == 0) {
> > > +                     rLenPtr = len1;
> > > +             } else if (rPx == 1) {
> > > +                     rLenPtr = len1;
> > > +             } else {
> > > +                     rLenPtr = lenFail;
> > > +                     rFail = 1;
> > > +             }
> > > +
> > > +             *rLenPtr = -1;
> > > +             smp_store_release(px, rPx + 1);
> > > +
> > > +             spin_unlock(rb_lock);
> > > +
> > > +             smp_store_release(rLenPtr, 1);
> >
> > I ran a test replacing the last 2 statements above with the following and it
> > still works:
> >
> >                 spin_unlock(rb_lock);
> >                 WRITE_ONCE(*rLenPtr, 1);
> >
> > Wouldn't you expect the test to catch an issue? The spin_unlock is already a
> > RELEASE barrier.
> 
> Well, apparently it's not an issue and WRITE_ONCE would work as well
> :) My original version actually used WRITE_ONCE here. See [0] and
> discussion in [1] after which I removed all the WRITE_ONCE/READ_ONCE
> in favor of store_release/load_acquire for consistency.
> 
>   [0] https://patchwork.ozlabs.org/project/netdev/patch/[email-masked]/
>   [1] https://patchwork.ozlabs.org/project/netdev/patch/[email-masked]/

Huh. So you are replacing the test to use WRITE_ONCE instead? Why did you
favor the acquire/release memory barriers over the _ONCE annotations, if that
was not really needed then?

> > Suggestion: It is hard to review the patch because it is huge, it would be
> > good to split this up into 4 patches for each of the tests. But upto you :)
> 
> Those 4 files are partial copies of each other, not sure splitting
> them actually would be easier. If anyone else thinks the same, though,
> I'll happily split.

I personally disagree. It would be much easier IMHO to review 4 different
files since some of them are also quite dissimilar. I frequently keep jumping
between diffs to find a different file and it makes the review that much
harder. But anything the LKMM experts decide in this regard is acceptable to me :)

thanks,

 - Joel