Re: [Qemu-devel] [PATCH 1/2] iothread: stash thread ID away

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/02/2014 15:51, Stefan Hajnoczi ha scritto:
> Keep the thread ID around so we can report it via QMP.
>
> There's only one problem: qemu_get_thread_id() (gettid() wrapper on
> Linux) must be called from the thread itself.  There is no way to get
> the thread ID outside the thread.
>
> This patch uses a condvar to wait for iothread_run() to populate the
> thread_id inside the thread.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  iothread.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/iothread.c b/iothread.c
> index 033de7f..1a0d1ec 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -26,8 +26,15 @@ struct IOThread {
>      QemuThread thread;
>      AioContext *ctx;
>      bool stopping;
> +    int thread_id;
>  };
>
> +typedef struct {
> +    QemuMutex init_done_lock;
> +    QemuCond init_done_cond;    /* is thread initialization done? */
> +    IOThread *iothread;
> +} ThreadInitInfo;
> +
>  #define IOTHREAD_GET_CLASS(obj) \
>     OBJECT_GET_CLASS(IOThreadClass, obj, TYPE_IOTHREAD)
>  #define IOTHREAD_CLASS(klass) \
> @@ -35,7 +42,15 @@ struct IOThread {
>
>  static void *iothread_run(void *opaque)
>  {
> -    IOThread *iothread = opaque;
> +    ThreadInitInfo *init_info = opaque;
> +    IOThread *iothread = init_info->iothread;
> +
> +    iothread->thread_id = qemu_get_thread_id();
> +
> +    /* Signal that initialization is done */
> +    qemu_mutex_lock(&init_info->init_done_lock);
> +    qemu_cond_signal(&init_info->init_done_cond);
> +    qemu_mutex_unlock(&init_info->init_done_lock);
>
>      while (!iothread->stopping) {
>          aio_context_acquire(iothread->ctx);
> @@ -60,15 +75,30 @@ static void iothread_instance_finalize(Object *obj)
>  static void iothread_complete(UserCreatable *obj, Error **errp)
>  {
>      IOThread *iothread = IOTHREAD(obj);
> +    ThreadInitInfo init_info = {
> +        .iothread = iothread,
> +    };
>
>      iothread->stopping = false;
>      iothread->ctx = aio_context_new();
>
> +    qemu_mutex_init(&init_info.init_done_lock);
> +    qemu_cond_init(&init_info.init_done_cond);
> +
>      /* This assumes we are called from a thread with useful CPU affinity for us
>       * to inherit.
>       */
>      qemu_thread_create(&iothread->thread, iothread_run,
> -                       iothread, QEMU_THREAD_JOINABLE);
> +                       &init_info, QEMU_THREAD_JOINABLE);
> +
> +    /* Wait for initialization to complete */
> +    qemu_mutex_lock(&init_info.init_done_lock);
> +    qemu_cond_wait(&init_info.init_done_cond,
> +                   &init_info.init_done_lock);
> +    qemu_mutex_unlock(&init_info.init_done_lock);
> +
> +    qemu_cond_destroy(&init_info.init_done_cond);
> +    qemu_mutex_destroy(&init_info.init_done_lock);

Destroying the mutex here is racy.  You need to keep it until the 
iothread is destroyed.

Paolo

>  }
>
>  static void iothread_class_init(ObjectClass *klass, void *class_data)
>