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Subject: Re: [PATCH v4] packed-ring: fix used descriptor checking in example code
On Thu, 6 Dec 2018 00:27:01 +0800 Tiwei Bie <tiwei.bie@intel.com> wrote: > When the driver is processing used descriptors in parallel > with adding new available descriptors, the driver can't just > check whether USED bit equals to the used wrap counter when > checking whether a descriptor is a used descriptor, because > the driver also needs to check whether the descriptor has > been made available. 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 the 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 the 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 the 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 the device made that descriptor used, the ring will be: > > +-----------+----+----+----+ > | 00 (id=0) | 10 | 11 | 10 | > +-----------+----+----+----+ > > If the 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. > > 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; > > v3: > - Compare with vq->used_wrap_count (MST); > - Add comments (MST); > - Refine commit log; > > v4: > - s/device/the device/ (Cornelia); > - s/driver/the driver/ (Cornelia); > - Improve commit log and comments (MST); > - Make commit log title more specific; > > packed-ring.tex | 23 ++++++++++++++++++++--- > 1 file changed, 20 insertions(+), 3 deletions(-) > > diff --git a/packed-ring.tex b/packed-ring.tex > index f24f49b..0047ecf 100644 > --- a/packed-ring.tex > +++ b/packed-ring.tex > @@ -687,16 +687,33 @@ vq->driver_event.flags = RING_EVENT_FLAGS_DISABLE; > for (;;) { > struct pvirtq_desc *d = vq->desc[vq->next_used]; > > + /* > + * Check that > + * 1. Descriptor has been made available. This check is necessary > + * if the driver is making new descriptors available in parallel > + * with this processing of used descriptors (e.g. from another thread). > + * Note: there are many other ways to check this, e.g. > + * track the number of outstanding available descriptors or buffers > + * and check that it's not 0. > + * 2. Descriptor has been used by the device. > + */ > 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 != vq->used_wrap_count || used != vq->used_wrap_count) { > vq->driver_event.flags = RING_EVENT_FLAGS_ENABLE; > memory_barrier(); > > + /* > + * Re-test in case the driver made more descriptors available in > + * parallel with the used descriptor processing (e.g. from another > + * thread) and/or the device used more descriptors before the driver > + * enabled events. > + */ > 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 != vq->used_wrap_count || used != vq->used_wrap_count) { > break; > } > Reviewed-by: Cornelia Huck <cohuck@redhat.com>
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