[dpdk-dev] [memnic PATCH] linux: fix race condition

[Hiroshi Shimamoto]

From: Hiroshi Shimamoto <h-shimamoto at ct.jp.nec.com>

Same as PMD, there is a race condition between host and guest
in the guest MEMNIC kernel driver.

Guest PMD                 Host
Thread-A     Thread-B     vSwitch
   |idx=0       |idx=0       |p[0] st!=2
   |cmpxchg     |            |
   |p[0] st->1  |            |
   |idx=1       |            |
   |fill data   |            |
   |p[0] st->2  |            |p[0] st==2
   |            |            |receive data
   |            |            |p[0] st->0
   |            |cmpxchg     |
   |            |success     |p[1] st!=2
   |            |p[0] st->1  |
                  This is BAD

Signed-off-by: Hiroshi Shimamoto <h-shimamoto at ct.jp.nec.com>
---
 linux/memnic_net.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/linux/memnic_net.c b/linux/memnic_net.c
index 9019258..fadece6 100644
--- a/linux/memnic_net.c
+++ b/linux/memnic_net.c
@@ -209,6 +209,15 @@ retry:
 		goto retry;
 	}
 
+	if (idx != ACCESS_ONCE(memnic->down)) {
+		/*
+		 * host freed this and got false positive,
+		 * need to recover the status and retry.
+		 */
+		p->status = MEMNIC_PKT_ST_FREE;
+		goto retry;
+	}
+
 	if (++idx >= MEMNIC_NR_PACKET)
 		idx = 0;
 	memnic->down = idx;
-- 
1.8.4