Re: [PATCH v2] packed-ring: fix example code

[Tiwei Bie <tiwei.bie@intel.com>]

On Tue, Dec 04, 2018 at 12:11:38PM -0500, Michael S. Tsirkin wrote:
> On Tue, Dec 04, 2018 at 06:49:29PM +0800, Tiwei Bie wrote:
> > Driver can't just check whether USED bit equals to the used
> > wrap counter when checking whether a descriptor is a used
> > descriptor. Below is an example:
> > 
> > Assuming ring size is 4, ring's initial state will be:
> > 
> > +----+----+----+----+
> > | 00 | 00 | 00 | 00 |
> > +----+----+----+----+
> > 
> > 00 means AVAIL=0 USED=0, 01 means AVAIL=0 USED=1
> > 10 means AVAIL=1 USED=0, 11 means AVAIL=1 USED=1
> > 
> > After driver made two descriptor chains available and each
> > chain consists of two descriptors, the ring could be:
> > 
> > +----+-----------+----+-----------+
> > | 10 | 10 (id=0) | 10 | 10 (id=1) |
> > +----+-----------+----+-----------+
> > 
> > After device processed all the available descriptors and made
> > them used (e.g. in order), the ring could be:
> > 
> > +-----------+----+-----------+----+
> > | 11 (id=0) | 10 | 11 (id=1) | 10 |
> > +-----------+----+-----------+----+
> > 
> > After driver processed all the used descriptors and made
> > one descriptor (not chained, just one descriptor) available,
> > the ring could be:
> > 
> > +-----------+----+----+----+
> > | 01 (id=0) | 10 | 11 | 10 |
> > +-----------+----+----+----+
> > 
> > After device made that descriptor used, the ring will be:
> > 
> > +-----------+----+----+----+
> > | 00 (id=0) | 10 | 11 | 10 |
> > +-----------+----+----+----+
> > 
> > If driver just checks whether USED bit equals to the used
> > wrap counter when checking whether a descriptor is a used
> > descriptor, after processing the first descriptor (whose
> > AVAIL and USED bits are both 0), and advancing vq->next_used
> > pointer, it will then also treat the next descriptor, i.e.
> > the second descriptor (whose AVAIL and USED bits are 1 and
> > 0 respectively) as a used descriptor which is wrong. The
> > most straightforward way to fix it is to also check whether
> > AVAIL bit equals to USED bit.
> > 
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/29
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > ---
> > v2:
> > - Add "Fixes" tag;
> > - Refine commit log;
> > 
> >  packed-ring.tex | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/packed-ring.tex b/packed-ring.tex
> > index f24f49b..c913768 100644
> > --- a/packed-ring.tex
> > +++ b/packed-ring.tex
> > @@ -688,15 +688,17 @@ for (;;) {
> >          struct pvirtq_desc *d = vq->desc[vq->next_used];
> >  
> >          flags = d->flags;
> > +        bool avail = flags & VIRTQ_DESC_F_AVAIL;
> >          bool used = flags & VIRTQ_DESC_F_USED;
> >  
> > -        if (used != vq->used_wrap_count) {
> > +        if (avail != used || used != vq->used_wrap_count) {
> 
> I think I prefer avail != vq->used_wrap_count.
> This is imho clearer because both bits are controlled by the driver.
> 
> And maybe add a comment along the lines of
> 
> /*
>  * Check that
>  * 1. Descriptor has been made available.
>  *    Note: there are many other way to check this, e.g.
>  *    track number of outstanding available descriptors or buffers
>  *    and check that it's not 0.
>  * 2. Descriptor has been used by device.
>  */
> 

OK.

> >                  vq->driver_event.flags = RING_EVENT_FLAGS_ENABLE;
> >                  memory_barrier();
> >  
> >                  flags = d->flags;
> > +                bool avail = flags & VIRTQ_DESC_F_AVAIL;
> >                  bool used = flags & VIRTQ_DESC_F_USED;
> > -                if (used != vq->used_wrap_count) {
> > +                if (avail != used || used != vq->used_wrap_count) {
> 
> So why do we need to re-test avail here? I guess it's in case
> driver is making descriptors available in parallel?
> If so maybe explain that:
> 
> /* 
>  * Re-test in case another thread submitted more descriptors
>  * and/or device used more descriptors before driver enabled events.
>  */
> 

OK.

Thanks

> 
> >                          break;
> >                  }
> 
> 
> >  
> > -- 
> > 2.17.1