[PATCH v3] packed-ring: fix example code

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

Driver can't just check whether USED bit equals to the used
wrap counter when checking whether a descriptor is a used
descriptor, because 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 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.

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;

 packed-ring.tex | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/packed-ring.tex b/packed-ring.tex
index f24f49b..c8cd83b 100644
--- a/packed-ring.tex
+++ b/packed-ring.tex
@@ -687,16 +687,29 @@ 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.
+         *    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 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 another thread submitted more descriptors
+                 * and/or device used more descriptors before 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;
                 }
 
-- 
2.17.1