[Qemu-devel] [PATCH 2/4] vnc: don't mess up with iohandlers in the vnc thread

[Corentin Chary]

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 1dfa6c3..4daf4b4 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 1b68965..5d4beaa 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
@@ -2521,6 +2532,7 @@ 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 f10c5dc..5599a4d 100644
--- a/ui/vnc.h
+++ b/ui/vnc.h
@@ -286,6 +286,8 @@ 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