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

[wangyunjian]

> -----Original Message-----
> From: Ferruh Yigit [mailto:ferruh.yigit at intel.com]
> Sent: Tuesday, June 22, 2021 8:28 PM
> To: wangyunjian <wangyunjian at huawei.com>; dev at dpdk.org
> Cc: thomas at monjalon.net; gowrishankar.m at linux.vnet.ibm.com;
> dingxiaoxiong <dingxiaoxiong at huawei.com>; stable at dpdk.org; liucheng (J)
> <liucheng11 at huawei.com>
> Subject: Re: [PATCH v2] kni: fix wrong mbuf alloc count in kni_allocate_mbufs
> 
> On 6/22/2021 11:57 AM, wangyunjian wrote:
> > 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>
> 
> Acked-by: Ferruh Yigit <ferruh.yigit at intel.com>
> 
> What do you think to change patch title to something like:
> kni: fix mbuf allocation for alloc FIFO

OK, I will change patch title later.

Thanks