Re: [dpdk-dev] [PATCH v2 02/10] kni: separate releasing netdev from freeing KNI interface

[Dan Gora <dg@adax.com>]

Hi Ferruh,

I remembered now the motivation behind separating rte_kni_release()
and rte_kni_free().

The problem is that the DPDK thread which calls rte_kni_release()
_cannot_ be the same thread which handles callbacks from the KNI
driver via rte_kni_handle_request().  This is because the thread which
calls rte_kni_release() will be stuck down in
ioctl(RTE_KNI_IOCTL_RELEASE) when the kernel calls the
RTE_KNI_REQ_CFG_NETWORK_IF callback to the DPDK application.  Since
that thread cannot call rte_kni_handle_request(), the callback would
then just timeout unless some other thread calls
rte_kni_handle_request().

So then you are in a bit of a chicken and egg situation.  You _have_
to have a separate thread calling rte_kni_handle_request periodically,
but that thread also _cannot_ run after rte_kni_release returns
(actually it's worse than that because it's actually after the
ioctl(RTE_KNI_IOCTL_RELEASE) returns and the fifos are freed).

So in order to resolve this, I separated the release from the freeing
stages. This allows the DPDK application to keep the
rte_kni_handle_request() thread running while rte_kni_release() is
called so that it can handle the interface state callback, then kill
that thread so that it cannot touch any 'struct rte_kni' resources,
then free the struct rte_kni resources.


thanks
dan

On Wed, Aug 29, 2018 at 7:59 AM, Ferruh Yigit <ferruh.yigit@intel.com> wrote:

>> When the kernel network interface is removed with unregister_netdev(),
>> if the interface is up, it will generate a callback to mark the
>> interface down, which calls kni_net_release().  kni_net_release() will
>> block waiting for the DPDK application to call rte_kni_handle_request()
>> to handle the callback, but it also needs the thread in the KNI driver
>> (either the per-dev thread for multi-thread or the per-driver thread)
>> to call kni_net_poll_resp() in order to wake the thread sleeping in
>> kni_net_release (actually kni_net_process_request()).
>>
>> So now, KNI interfaces should be removed as such:
>>
>> 1) The user calls rte_kni_release().  This only unregisters the
>> netdev in the kernel, but touches nothing else.  This allows all the
>> threads to run which are necessary to handle the callback into the
>> DPDK application to mark the interface down.
>>
>> 2) The user stops the thread running rte_kni_handle_request().
>> After rte_kni_release() has been called, there will be no more
>> callbacks for that interface so it is not necessary.  It cannot be
>> running at the same time that rte_kni_free() frees all of the FIFOs
>> and DPDK memory for that KNI interface.
>>
>> 3) The user calls rte_kni_free().  This performs the RTE_KNI_IOCTL_FREE
>> ioctl which calls kni_ioctl_free().  This function removes the struct
>> kni_dev from the list of interfaces to poll (and kills the per-dev
>> kthread, if configured for multi-thread), then frees the memory in
>> the FIFOs.