RE: [PATCH] multifd: ensure multifd threads are terminated before cleanup params

[Wangxin (Alexander)]

Ping.

> 
> In multifd_save_cleanup(), we terminate all multifd threads and destroy
> the 'p->mutex', while the mutex may still be held by multifd send thread,
> this causes qemu to crash.
> 
> It's because the multifd_send_thread maybe scheduled out after setting
> 'p->running' to false. To reproduce the problem, we put
> 'multifd_send_thread' to sleep seconds before unlock 'p->mutex':
> 
> function multifd_send_thread()
> {
>     ...
>     qemu_mutex_lock(&p->mutex);
>     p->running = false;
>     usleep(5000000);
>     ^^^^^^^^^^^^^^^^
>     qemu_mutex_unlock(&p->mutex);
>     ...
> }
> 
> As the 'p->running' is used to indicate whether the multifd_send/recv thread
> is created, it should be set to false after the thread terminate.
> 
> Signed-off-by: Wang Xin <wangxinxin.wang@huawei.com>
> Signed-off-by: Huangyu Zhai <zhaihuanyu@huawei.com>
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 76b57a7177..d8fc7d319e 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -526,6 +526,7 @@ void multifd_save_cleanup(void)
> 
>          if (p->running) {
>              qemu_thread_join(&p->thread);
> +            p->running = false;
>          }
>      }
>      for (i = 0; i < migrate_multifd_channels(); i++) {
> @@ -707,10 +708,6 @@ out:
>          qemu_sem_post(&multifd_send_state->channels_ready);
>      }
> 
> -    qemu_mutex_lock(&p->mutex);
> -    p->running = false;
> -    qemu_mutex_unlock(&p->mutex);
> -
>      rcu_unregister_thread();
>      trace_multifd_send_thread_end(p->id, p->num_packets, 
> p->total_normal_pages);
> 
> @@ -995,6 +992,7 @@ int multifd_load_cleanup(Error **errp)
>               */
>              qemu_sem_post(&p->sem_sync);
>              qemu_thread_join(&p->thread);
> +            p->running = false;
>          }
>      }
>      for (i = 0; i < migrate_multifd_channels(); i++) {
> @@ -1110,9 +1108,6 @@ static void *multifd_recv_thread(void *opaque)
>          multifd_recv_terminate_threads(local_err);
>          error_free(local_err);
>      }
> -    qemu_mutex_lock(&p->mutex);
> -    p->running = false;
> -    qemu_mutex_unlock(&p->mutex);
> 
>      rcu_unregister_thread();
>      trace_multifd_recv_thread_end(p->id, p->num_packets, 
> p->total_normal_pages);
> --
> 2.26.0.windows.1