Re: [dpdk-dev] [ovs-dev] [PATCH] vhost: return -EAGAIN during unregistering vhost if it is busy.

[王志克 <wangzk320@163.com>]

NO, it is different issue.


The current  deadlock  mentioned in this patch is caused by some blocking function (like ovsrcu_synchronize) in application (like OVS).
In this patch, the application is needed  to break  the logical deadlock. 

















At 2020-04-27 16:09:31, "Maxime Coquelin" <maxime.coquelin@redhat.com> wrote:
>
>
>On 3/18/20 4:31 AM, 王志克 wrote:
>> Involve openvswitch group since this fix is highly coupled with OVS.
>> welcome comment.
>> At 2020-03-12 17:57:19, "Zhike Wang" <wangzhike@jd.com> wrote:
>>> The vhost_user_read_cb() and rte_vhost_driver_unregister()
>>> can be called at the same time by 2 threads, and may lead to deadlock.
>>> Eg thread1 calls vhost_user_read_cb()->vhost_user_get_vring_base()->destroy_device(),
>>> then thread2 calls rte_vhost_driver_unregister(), and will retry the fdset_try_del() in loop.
>>>
>>> Some application implements destroy_device() as a blocking function, eg
>>> OVS calls ovsrcu_synchronize() insides destroy_device(). As a result,
>>> thread1(eg vhost_events) is blocked to wait quiesce of thread2(eg ovs-vswitchd),
>>> and thread2 is in a loop to wait thread1 to give up the use of the vhost fd,
>>> then leads to deadlock.
>>>
>>> It is better to return -EAGAIN to application, who will decide how to handle
>>> (eg OVS can call ovsrcu_quiesce() and then retry).
>>>
>>> Signed-off-by: Zhike Wang <wangzhike@jd.com>
>>> ---
>>> lib/librte_vhost/rte_vhost.h | 4 +++-
>>> lib/librte_vhost/socket.c    | 8 ++++----
>>> 2 files changed, 7 insertions(+), 5 deletions(-)
>
>
>Isn't it fixed with below commit that landed into DPDK v20.02?
>
>commit 5efb18e85f7fdb436d3e56591656051c16802066
>Author: Maxime Coquelin <maxime.coquelin@redhat.com>
>Date:   Tue Jan 14 19:53:57 2020 +0100
>
>    vhost: fix deadlock on port deletion
>
>    If the vhost-user application (e.g. OVS) deletes the vhost-user
>    port while Qemu sends a vhost-user request, a deadlock can
>    happen if the request handler tries to acquire vhost-user's
>    global mutex, which is also locked by the vhost-user port
>    deletion API (rte_vhost_driver_unregister).
>
>    This patch prevents the deadlock by making
>    rte_vhost_driver_unregister() to release the mutex and try
>    again if a request is being handled to give a chance to
>    the request handler to complete.
>
>    Fixes: 8b4b949144b8 ("vhost: fix dead lock on closing in server mode")
>    Fixes: 5fbb3941da9f ("vhost: introduce driver features related APIs")
>    Cc: stable@dpdk.org
>
>    Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>    Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
>    Acked-by: Eelco Chaudron <echaudro@redhat.com>
>
>>> diff --git a/lib/librte_vhost/rte_vhost.h b/lib/librte_vhost/rte_vhost.h
>>> index c7b619a..276db11 100644
>>> --- a/lib/librte_vhost/rte_vhost.h
>>> +++ b/lib/librte_vhost/rte_vhost.h
>>> @@ -389,7 +389,9 @@ void rte_vhost_log_used_vring(int vid, uint16_t vring_idx,
>>>  */
>>> int rte_vhost_driver_register(const char *path, uint64_t flags);
>>>
>>> -/* Unregister vhost driver. This is only meaningful to vhost user. */
>>> +/* Unregister vhost driver. This is only meaningful to vhost user.
>>> + * Return -EAGAIN if device is busy, and leave it to be handled by application.
>>> + */
>>> int rte_vhost_driver_unregister(const char *path);
>>>
>>> /**
>>> diff --git a/lib/librte_vhost/socket.c b/lib/librte_vhost/socket.c
>>> index 7c80121..a75a3f6 100644
>>> --- a/lib/librte_vhost/socket.c
>>> +++ b/lib/librte_vhost/socket.c
>>> @@ -1027,7 +1027,8 @@ struct vhost_user_reconnect_list {
>>> }
>>>
>>> /**
>>> - * Unregister the specified vhost socket
>>> + * Unregister the specified vhost socket.
>>> + * Return -EAGAIN if device is busy, and leave it to be handled by application.
>>>  */
>>> int
>>> rte_vhost_driver_unregister(const char *path)
>>> @@ -1039,7 +1040,6 @@ struct vhost_user_reconnect_list {
>>> 	if (path == NULL)
>>> 		return -1;
>>>
>>> -again:
>>> 	pthread_mutex_lock(&vhost_user.mutex);
>>>
>>> 	for (i = 0; i < vhost_user.vsocket_cnt; i++) {
>>> @@ -1063,7 +1063,7 @@ struct vhost_user_reconnect_list {
>>> 					pthread_mutex_unlock(
>>> 							&vsocket->conn_mutex);
>>> 					pthread_mutex_unlock(&vhost_user.mutex);
>>> -					goto again;
>>> +					return -EAGAIN;
>>> 				}
>>>
>>> 				VHOST_LOG_CONFIG(INFO,
>>> @@ -1085,7 +1085,7 @@ struct vhost_user_reconnect_list {
>>> 				if (fdset_try_del(&vhost_user.fdset,
>>> 						vsocket->socket_fd) == -1) {
>>> 					pthread_mutex_unlock(&vhost_user.mutex);
>>> -					goto again;
>>> +					return -EAGAIN;
>>> 				}
>>>
>>> 				close(vsocket->socket_fd);
>>> -- 
>>> 1.8.3.1
>>>
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