diff options
| -rw-r--r-- | packed-ring.tex | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/packed-ring.tex b/packed-ring.tex index 4395b5a..e10d87f 100644 --- a/packed-ring.tex +++ b/packed-ring.tex @@ -685,16 +685,33 @@ 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; } |