Author: vmaffione
Date: Wed Jun 10 20:05:07 2020
New Revision: 362025
URL: https://svnweb.freebsd.org/changeset/base/362025
Log:
MFC r361759
netmap: vtnet: fix race condition in rxsync
This change prevents a race that happens when rxsync dequeues
N-1 rx packets (with N being the size of the netmap rx ring).
In this situation, the loop exits without re-enabling the
rx interrupts, thus causing the VQ to stall.
Modified:
stable/12/sys/dev/netmap/if_vtnet_netmap.h
Directory Properties:
stable/12/ (props changed)
Modified: stable/12/sys/dev/netmap/if_vtnet_netmap.h
==============================================================================
--- stable/12/sys/dev/netmap/if_vtnet_netmap.h Wed Jun 10 20:04:20 2020
(r362024)
+++ stable/12/sys/dev/netmap/if_vtnet_netmap.h Wed Jun 10 20:05:07 2020
(r362025)
@@ -310,8 +310,11 @@ vtnet_netmap_rxsync(struct netmap_kring *kring, int fl
/*
* First part: import newly received packets.
* Only accept our own buffers (matching the token). We should only get
- * matching buffers. We may need to stop early to avoid hwtail to
overrun
- * hwcur.
+ * matching buffers. The hwtail should never overrun hwcur, because
+ * we publish only N-1 receive buffers (and non N).
+ * In any case we must not leave this routine with the interrupts
+ * disabled, pending packets in the VQ and hwtail == (hwcur - 1),
+ * otherwise the pending packets could stall.
*/
if (netmap_no_pendintr || force_update) {
uint32_t hwtail_lim = nm_prev(kring->nr_hwcur, lim);
@@ -320,10 +323,17 @@ vtnet_netmap_rxsync(struct netmap_kring *kring, int fl
vtnet_rxq_disable_intr(rxq);
nm_i = kring->nr_hwtail;
- while (nm_i != hwtail_lim) {
+ for (;;) {
int len;
token = virtqueue_dequeue(vq, &len);
if (token == NULL) {
+ /*
+ * Enable the interrupts again and double-check
+ * for more work. We can go on until we win the
+ * race condition, since we are not replenishing
+ * in the meanwhile, and thus we will process at
+ * most N-1 slots.
+ */
if (interrupts && vtnet_rxq_enable_intr(rxq)) {
vtnet_rxq_disable_intr(rxq);
continue;
@@ -333,6 +343,11 @@ vtnet_netmap_rxsync(struct netmap_kring *kring, int fl
if (unlikely(token != (void *)rxq)) {
nm_prerr("BUG: RX token mismatch");
} else {
+ if (nm_i == hwtail_lim) {
+ KASSERT(false, ("hwtail would "
+ "overrun hwcur"));
+ }
+
/* Skip the virtio-net header. */
len -= sc->vtnet_hdr_size;
if (unlikely(len < 0)) {
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