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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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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