[dpdk-stable] [PATCH v2] kni: fix wrong mbuf alloc count in kni_allocate_mbufs

[wangyunjian]

From: Yunjian Wang <wangyunjian at huawei.com>

In kni_allocate_mbufs(), we alloc mbuf for alloc_q as this code.
allocq_free = (kni->alloc_q->read - kni->alloc_q->write - 1) \
		& (MAX_MBUF_BURST_NUM - 1);
The value of allocq_free maybe zero, for example :
The ring size is 1024. After init, write = read = 0. Then we fill
kni->alloc_q to full. At this time, write = 1023, read = 0.

Then the kernel send 32 packets to userspace. At this time, write
= 1023, read = 32. And then the userspace receive this 32 packets.
Then fill the kni->alloc_q, (32 - 1023 - 1) & 31 = 0, fill nothing.
...
Then the kernel send 32 packets to userspace. At this time, write
= 1023, read = 992. And then the userspace receive this 32 packets.
Then fill the kni->alloc_q, (992 - 1023 - 1) & 31 = 0, fill nothing.

Then the kernel send 32 packets to userspace. The kni->alloc_q only
has 31 mbufs and will drop one packet.

Absolutely, this is a special scene. Normally, it will fill some
mbufs everytime, but may not enough for the kernel to use.

In this patch, we always keep the kni->alloc_q to full for the kernel
to use.

Fixes: 49da4e82cf94 ("kni: allocate no more mbuf than empty slots in queue")
Cc: stable at dpdk.org

Signed-off-by: Cheng Liu <liucheng11 at huawei.com>
Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
---
v2:
   add fixes tag and update commit log
---
 lib/kni/rte_kni.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/kni/rte_kni.c b/lib/kni/rte_kni.c
index 9dae6a8d7c..eb24b0d0ae 100644
--- a/lib/kni/rte_kni.c
+++ b/lib/kni/rte_kni.c
@@ -677,8 +677,9 @@ kni_allocate_mbufs(struct rte_kni *kni)
 		return;
 	}
 
-	allocq_free = (kni->alloc_q->read - kni->alloc_q->write - 1)
-			& (MAX_MBUF_BURST_NUM - 1);
+	allocq_free = kni_fifo_free_count(kni->alloc_q);
+	allocq_free = (allocq_free > MAX_MBUF_BURST_NUM) ?
+		MAX_MBUF_BURST_NUM : allocq_free;
 	for (i = 0; i < allocq_free; i++) {
 		pkts[i] = rte_pktmbuf_alloc(kni->pktmbuf_pool);
 		if (unlikely(pkts[i] == NULL)) {
-- 
2.23.0