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Subject: Re: [PATCH v2] packed-ring: fix example code
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. */ > 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. */ > break; > } > > -- > 2.17.1
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