[Qemu-devel] Re: [PATCH v5] vnc: don't mess up with iohandlers in the vnc thread

[Corentin Chary]

On Thu, Mar 10, 2011 at 3:13 PM, Corentin Chary
<address@hidden> wrote:
> The threaded VNC servers messed up with QEMU fd handlers without
> any kind of locking, and that can cause some nasty race conditions.
>
> Using qemu_mutex_lock_iothread() won't work because vnc_dpy_cpy(),
> which will wait for the current job queue to finish, can be called with
> the iothread lock held.
>
> Instead, we now store the data in a temporary buffer, and use a bottom
> half to notify the main thread that new data is available.
>
> vnc_[un]lock_ouput() is still needed to access VncState members like
> abort, csock or jobs_buffer.
>
> Signed-off-by: Corentin Chary <address@hidden>
> ---
>  ui/vnc-jobs-async.c |   48 +++++++++++++++++++++++++++++-------------------
>  ui/vnc-jobs.h       |    1 +
>  ui/vnc.c            |   12 ++++++++++++
>  ui/vnc.h            |    2 ++
>  4 files changed, 44 insertions(+), 19 deletions(-)
>
> diff --git a/ui/vnc-jobs-async.c b/ui/vnc-jobs-async.c
> index f596247..4438776 100644
> --- a/ui/vnc-jobs-async.c
> +++ b/ui/vnc-jobs-async.c
> @@ -28,6 +28,7 @@
>
>  #include "vnc.h"
>  #include "vnc-jobs.h"
> +#include "qemu_socket.h"
>
>  /*
>  * Locking:
> @@ -155,6 +156,24 @@ void vnc_jobs_join(VncState *vs)
>         qemu_cond_wait(&queue->cond, &queue->mutex);
>     }
>     vnc_unlock_queue(queue);
> +    vnc_jobs_consume_buffer(vs);
> +}
> +
> +void vnc_jobs_consume_buffer(VncState *vs)
> +{
> +    bool flush;
> +
> +    vnc_lock_output(vs);
> +    if (vs->jobs_buffer.offset) {
> +        vnc_write(vs, vs->jobs_buffer.buffer, vs->jobs_buffer.offset);
> +        buffer_reset(&vs->jobs_buffer);
> +    }
> +    flush = vs->csock != -1 && vs->abort != true;
> +    vnc_unlock_output(vs);
> +
> +    if (flush) {
> +      vnc_flush(vs);
> +    }
>  }
>
>  /*
> @@ -197,7 +216,6 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
>     VncState vs;
>     int n_rectangles;
>     int saved_offset;
> -    bool flush;
>
>     vnc_lock_queue(queue);
>     while (QTAILQ_EMPTY(&queue->jobs) && !queue->exit) {
> @@ -213,6 +231,7 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
>
>     vnc_lock_output(job->vs);
>     if (job->vs->csock == -1 || job->vs->abort == true) {
> +        vnc_unlock_output(job->vs);
>         goto disconnected;
>     }
>     vnc_unlock_output(job->vs);
> @@ -233,10 +252,6 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
>
>         if (job->vs->csock == -1) {
>             vnc_unlock_display(job->vs->vd);
> -            /* output mutex must be locked before going to
> -             * disconnected:
> -             */
> -            vnc_lock_output(job->vs);
>             goto disconnected;
>         }
>
> @@ -254,24 +269,19 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
>     vs.output.buffer[saved_offset] = (n_rectangles >> 8) & 0xFF;
>     vs.output.buffer[saved_offset + 1] = n_rectangles & 0xFF;
>
> -    /* Switch back buffers */
>     vnc_lock_output(job->vs);
> -    if (job->vs->csock == -1) {
> -        goto disconnected;
> +    if (job->vs->csock != -1) {
> +        buffer_reserve(&job->vs->jobs_buffer, vs.output.offset);
> +        buffer_append(&job->vs->jobs_buffer, vs.output.buffer,
> +                      vs.output.offset);
> +        /* Copy persistent encoding data */
> +        vnc_async_encoding_end(job->vs, &vs);
> +
> +       qemu_bh_schedule(job->vs->bh);
>     }
> -
> -    vnc_write(job->vs, vs.output.buffer, vs.output.offset);
> -
> -disconnected:
> -    /* Copy persistent encoding data */
> -    vnc_async_encoding_end(job->vs, &vs);
> -    flush = (job->vs->csock != -1 && job->vs->abort != true);
>     vnc_unlock_output(job->vs);
>
> -    if (flush) {
> -        vnc_flush(job->vs);
> -    }
> -
> +disconnected:
>     vnc_lock_queue(queue);
>     QTAILQ_REMOVE(&queue->jobs, job, next);
>     vnc_unlock_queue(queue);
> diff --git a/ui/vnc-jobs.h b/ui/vnc-jobs.h
> index b8dab81..4c661f9 100644
> --- a/ui/vnc-jobs.h
> +++ b/ui/vnc-jobs.h
> @@ -40,6 +40,7 @@ void vnc_jobs_join(VncState *vs);
>
>  #ifdef CONFIG_VNC_THREAD
>
> +void vnc_jobs_consume_buffer(VncState *vs);
>  void vnc_start_worker_thread(void);
>  bool vnc_worker_thread_running(void);
>  void vnc_stop_worker_thread(void);
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 610f884..f28873b 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -1011,7 +1011,10 @@ static void vnc_disconnect_finish(VncState *vs)
>
>  #ifdef CONFIG_VNC_THREAD
>     qemu_mutex_destroy(&vs->output_mutex);
> +    qemu_bh_delete(vs->bh);
> +    buffer_free(&vs->jobs_buffer);
>  #endif
> +
>     for (i = 0; i < VNC_STAT_ROWS; ++i) {
>         qemu_free(vs->lossy_rect[i]);
>     }
> @@ -1226,6 +1229,14 @@ static long vnc_client_read_plain(VncState *vs)
>     return ret;
>  }
>
> +#ifdef CONFIG_VNC_THREAD
> +static void vnc_jobs_bh(void *opaque)
> +{
> +    VncState *vs = opaque;
> +
> +    vnc_jobs_consume_buffer(vs);
> +}
> +#endif
>
>  /*
>  * First function called whenever there is more data to be read from
> @@ -2525,6 +2536,7 @@ static void vnc_connect(VncDisplay *vd, int csock)
>
>  #ifdef CONFIG_VNC_THREAD
>     qemu_mutex_init(&vs->output_mutex);
> +    vs->bh = qemu_bh_new(vnc_jobs_bh, vs);
>  #endif
>
>     QTAILQ_INSERT_HEAD(&vd->clients, vs, next);
> diff --git a/ui/vnc.h b/ui/vnc.h
> index 8a1e7b9..bca5f87 100644
> --- a/ui/vnc.h
> +++ b/ui/vnc.h
> @@ -283,6 +283,8 @@ struct VncState
>     VncJob job;
>  #else
>     QemuMutex output_mutex;
> +    QEMUBH *bh;
> +    Buffer jobs_buffer;
>  #endif
>
>     /* Encoding specific, if you add something here, don't forget to
> --
> 1.7.3.4
>
>

Paolo, Anthony, Jan, are you ok with that one ?
Peter Lieve, could you test that patch ?
Thanks

-- 
Corentin Chary
http://xf.iksaif.net