[dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v4

[Ferruh Yigit]

On 2/25/2021 2:32 PM, Elad Nachman wrote:
> This part of the series includes my fixes for the issues reported
> by Ferruh and Igor (and Igor comments for v3 of the patch)
> on top of part 1 of the patch series:
> 
> A. KNI sync lock is being locked while rtnl is held.
> If two threads are calling kni_net_process_request() ,
> then the first one will take the sync lock, release rtnl lock then sleep.
> The second thread will try to lock sync lock while holding rtnl.
> The first thread will wake, and try to lock rtnl, resulting in a deadlock.
> The remedy is to release rtnl before locking the KNI sync lock.
> Since in between nothing is accessing Linux network-wise,
> no rtnl locking is needed.
> 
> B. There is a race condition in __dev_close_many() processing the
> close_list while the application terminates.
> It looks like if two vEth devices are terminating,
> and one releases the rtnl lock, the other takes it,
> updating the close_list in an unstable state,
> causing the close_list to become a circular linked list,
> hence list_for_each_entry() will endlessly loop inside
> __dev_close_many() .
> Since the description for the original patch indicate the
> original motivation was bringing the device up,
> I have changed kni_net_process_request() to hold the rtnl mutex
> in case of bringing the device down since this is the path called
> from __dev_close_many() , causing the corruption of the close_list.
> In order to prevent deadlock in Mellanox device in this case, the
> code has been modified not to wait for user-space while holding
> the rtnl lock.
> Instead, after the request has been sent, all locks are relinquished
> and the function exits immediately with return code of zero (success).
> 
> To summarize:
> request != interface down : unlock rtnl, send request to user-space,
> wait for response, send the response error code to caller in user-space.
> 
> request == interface down: send request to user-space, return immediately
> with error code of 0 (success) to user-space.
> 
> Signed-off-by: Elad Nachman <eladv6 at gmail.com>
> 
> 
> ---
> v4:
> * for if down case, send asynchronously with rtnl locked and without
>    wait, returning immediately to avoid both kernel race conditions
>    and deadlock in user-space
> v3:
> * Include original patch and new patch as a series of patch, added a
>    comment to the new patch
> v2:
> * rebuild the patch as increment from patch 64106
> * fix comment and blank lines
> ---
>   kernel/linux/kni/kni_net.c      | 41 +++++++++++++++++++++++++++------
>   lib/librte_kni/rte_kni.c        |  7 ++++--
>   lib/librte_kni/rte_kni_common.h |  1 +
>   3 files changed, 40 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/linux/kni/kni_net.c b/kernel/linux/kni/kni_net.c
> index f0b6e9a8d..ba991802b 100644
> --- a/kernel/linux/kni/kni_net.c
> +++ b/kernel/linux/kni/kni_net.c
> @@ -110,12 +110,34 @@ kni_net_process_request(struct net_device *dev, struct rte_kni_request *req)
>   	void *resp_va;
>   	uint32_t num;
>   	int ret_val;
> +	int req_is_dev_stop = 0;
> +
> +	/* For configuring the interface to down,
> +	 * rtnl must be held all the way to prevent race condition
> +	 * inside __dev_close_many() between two netdev instances of KNI
> +	 */
> +	if (req->req_id == RTE_KNI_REQ_CFG_NETWORK_IF &&
> +			req->if_up == 0)
> +		req_is_dev_stop = 1;

Having this request type checks in the 'kni_net_process_request()' function 
looks like hack.
Since adding a new field into the "struct rte_kni_request", that can be a more 
generic 'asnyc' field, and the requested function, like 'kni_net_release()' can 
set it to support async requests.

And can you please separate the function to add a more generic async request 
support on its patch, which should do:
- add new 'asnyc' field to "struct rte_kni_request"
- in 'kni_net_process_request()', if 'req->async' set, do not wait for response
- in library, 'lib/librte_kni/rte_kni.c', in 'rte_kni_handle_request()' 
function, if the request is async don't put the response
(These are already done in this patch)

Overall it can be three patch set:
1) Function parameter change
2) Add generic async request support (with documentation update)
3) rtnl unlock and make 'kni_net_release()' request async (actual fix)
(We can discuss more if to make 'kni_net_release()' async with a kernel 
parameter or not)

What do you think, does it make sense?