Re: [Qemu-devel] [PATCH] iothread: Stop threads before main() quits

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Sep 08, 2016 at 05:28:51PM +0800, Fam Zheng wrote:
> Right after main_loop ends, we release various things but keep iothread
> alive. The latter is not prepared to the sudden change of resources.
> 
> Specifically, after bdrv_close_all(), virtio-scsi dataplane get a
> surprise at the empty BlockBackend:
> 
> (gdb) bt
>     at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:543
>     at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:577
> 
> It is because the d->conf.blk->root is set to NULL, then
> blk_get_aio_context() returns qemu_aio_context, whereas s->ctx is still
> pointing to the iothread:
> 
>     hw/scsi/virtio-scsi.c:543:
> 
>     if (s->dataplane_started) {
>         assert(blk_get_aio_context(d->conf.blk) == s->ctx);
>     }
> 
> To fix this, let's stop iothreads before doing bdrv_close_all().

Did you consider blk_add_remove_bs_notifier()?  It gets called during
bdrv_close_all() and would let virtio-scsi survive any other case where
the same thing happens.

This patch is fine for now but we should think about which approach to
take for all devices in the future.

> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  include/sysemu/iothread.h |  1 +
>  iothread.c                | 24 ++++++++++++++++++++----
>  vl.c                      |  2 ++
>  3 files changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h
> index 2eefea1..68ac2de 100644
> --- a/include/sysemu/iothread.h
> +++ b/include/sysemu/iothread.h
> @@ -35,5 +35,6 @@ typedef struct {
>  
>  char *iothread_get_id(IOThread *iothread);
>  AioContext *iothread_get_aio_context(IOThread *iothread);
> +void iothread_stop_all(void);
>  
>  #endif /* IOTHREAD_H */
> diff --git a/iothread.c b/iothread.c
> index f183d38..fb08a60 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -54,16 +54,25 @@ static void *iothread_run(void *opaque)
>      return NULL;
>  }
>  
> -static void iothread_instance_finalize(Object *obj)
> +static int iothread_stop(Object *object, void *opaque)
>  {
> -    IOThread *iothread = IOTHREAD(obj);
> +    IOThread *iothread;
>  
> -    if (!iothread->ctx) {
> -        return;
> +    iothread = (IOThread *)object_dynamic_cast(object, TYPE_IOTHREAD);
> +    if (!iothread || !iothread->ctx) {
> +        return 0;
>      }
>      iothread->stopping = true;
>      aio_notify(iothread->ctx);
>      qemu_thread_join(&iothread->thread);
> +    return 0;
> +}
> +
> +static void iothread_instance_finalize(Object *obj)
> +{
> +    IOThread *iothread = IOTHREAD(obj);
> +
> +    iothread_stop(obj, NULL);
>      qemu_cond_destroy(&iothread->init_done_cond);
>      qemu_mutex_destroy(&iothread->init_done_lock);
>      aio_context_unref(iothread->ctx);
> @@ -174,3 +183,10 @@ IOThreadInfoList *qmp_query_iothreads(Error **errp)
>      object_child_foreach(container, query_one_iothread, &prev);
>      return head;
>  }
> +
> +void iothread_stop_all(void)
> +{
> +    Object *container = object_get_objects_root();
> +
> +    object_child_foreach(container, iothread_stop, NULL);
> +}
> diff --git a/vl.c b/vl.c
> index ee557a1..6a218ce 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -121,6 +121,7 @@ int main(int argc, char **argv)
>  #include "crypto/init.h"
>  #include "sysemu/replay.h"
>  #include "qapi/qmp/qerror.h"
> +#include "sysemu/iothread.h"
>  
>  #define MAX_VIRTIO_CONSOLES 1
>  #define MAX_SCLP_CONSOLES 1
> @@ -4616,6 +4617,7 @@ int main(int argc, char **argv, char **envp)
>      trace_init_vcpu_events();
>      main_loop();
>      replay_disable_events();
> +    iothread_stop_all();
>  
>      bdrv_close_all();
>      pause_all_vcpus();
> -- 
> 2.7.4
> 

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