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

[Peter Lieven <pl@dlh.net>]

On 15.03.2011 17:55, Peter Lieven wrote:
> On 14.03.2011 10:19, Corentin Chary wrote:
>> On Thu, Mar 10, 2011 at 3:13 PM, Corentin Chary
>> <corentin.chary@gmail.com>  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<corentin.chary@gmail.com>
>>> ---
>>>   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 ?
> i have seen one segfault. unfortunatly, i had no debugger attached.
>
> but whats equally worse during stress test, i managed to trigger 
> oom-killer.
> it seems we have a memory leak somewhere....
commit c53af37f375ce9c4999ff451c51173bdc1167e67
seems to fix this. remember this is not in 0.14.0 which could cause
a lot of trouble to 0.14.0 users when they do a lot of vnc work...

peter



>
> peter
>
>
>> Thanks
>>
>