[Qemu-devel] [PATCH 2/2] 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 socket
pair to notify the main thread that new data is available. The data
is not directly send through this socket because it would make the code
more complex without any performance benefit.

vnc_[un]lock_ouput() is still needed to access VncState members like
abort, csock or jobs_buffer.

Thanks to Jan Kiszka for helping me solve this issue.

Signed-off-by: Corentin Chary <address@hidden>
---
 ui/vnc-jobs-async.c |   50 ++++++++++++++++++++++++++++++++------------------
 ui/vnc-jobs-sync.c  |    4 ++++
 ui/vnc-jobs.h       |    1 +
 ui/vnc.c            |   16 ++++++++++++++++
 ui/vnc.h            |    2 ++
 5 files changed, 55 insertions(+), 18 deletions(-)

diff --git a/ui/vnc-jobs-async.c b/ui/vnc-jobs-async.c
index f596247..543b5a9 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 +269,23 @@ 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;
-    }
-
-    vnc_write(job->vs, vs.output.buffer, vs.output.offset);
+    if (job->vs->csock != -1) {
+        int notify = !job->vs->jobs_buffer.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);
+        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);
 
-    if (flush) {
-        vnc_flush(job->vs);
+        if (notify) {
+            send(job->vs->jobs_socks[1], (char []){1}, 1, 0);
+        }
     }
+    vnc_unlock_output(job->vs);
 
+disconnected:
     vnc_lock_queue(queue);
     QTAILQ_REMOVE(&queue->jobs, job, next);
     vnc_unlock_queue(queue);
diff --git a/ui/vnc-jobs-sync.c b/ui/vnc-jobs-sync.c
index 49b77af..3a4d7df 100644
--- a/ui/vnc-jobs-sync.c
+++ b/ui/vnc-jobs-sync.c
@@ -36,6 +36,10 @@ void vnc_jobs_join(VncState *vs)
 {
 }
 
+void vnc_jobs_consume_buffer(VncState *vs)
+{
+}
+
 VncJob *vnc_job_new(VncState *vs)
 {
     vs->job.vs = vs;
diff --git a/ui/vnc-jobs.h b/ui/vnc-jobs.h
index b8dab81..7c529b7 100644
--- a/ui/vnc-jobs.h
+++ b/ui/vnc-jobs.h
@@ -37,6 +37,7 @@ void vnc_job_push(VncJob *job);
 bool vnc_has_job(VncState *vs);
 void vnc_jobs_clear(VncState *vs);
 void vnc_jobs_join(VncState *vs);
+void vnc_jobs_consume_buffer(VncState *vs);
 
 #ifdef CONFIG_VNC_THREAD
 
diff --git a/ui/vnc.c b/ui/vnc.c
index 610f884..48a81a2 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -1011,6 +1011,10 @@ static void vnc_disconnect_finish(VncState *vs)
 
 #ifdef CONFIG_VNC_THREAD
     qemu_mutex_destroy(&vs->output_mutex);
+    qemu_set_fd_handler2(vs->jobs_socks[0], NULL, NULL, NULL, vs);
+    close(vs->jobs_socks[0]);
+    close(vs->jobs_socks[1]);
+    buffer_free(&vs->jobs_buffer);
 #endif
     for (i = 0; i < VNC_STAT_ROWS; ++i) {
         qemu_free(vs->lossy_rect[i]);
@@ -1226,6 +1230,16 @@ static long vnc_client_read_plain(VncState *vs)
     return ret;
 }
 
+#ifdef CONFIG_VNC_THREAD
+static void vnc_jobs_read(void *opaque)
+{
+    VncState *vs = opaque;
+    char dummy;
+
+    recv(vs->jobs_socks[0], (void *)&dummy, 1, 0);
+    vnc_jobs_consume_buffer(vs);
+}
+#endif
 
 /*
  * First function called whenever there is more data to be read from
@@ -2525,6 +2539,8 @@ static void vnc_connect(VncDisplay *vd, int csock)
 
 #ifdef CONFIG_VNC_THREAD
     qemu_mutex_init(&vs->output_mutex);
+    qemu_socketpair(AF_UNIX, SOCK_STREAM, 0, vs->jobs_socks);
+    qemu_set_fd_handler2(vs->jobs_socks[0], NULL, vnc_jobs_read, NULL, vs);
 #endif
 
     QTAILQ_INSERT_HEAD(&vd->clients, vs, next);
diff --git a/ui/vnc.h b/ui/vnc.h
index 8a1e7b9..0f98f2e 100644
--- a/ui/vnc.h
+++ b/ui/vnc.h
@@ -283,6 +283,8 @@ struct VncState
     VncJob job;
 #else
     QemuMutex output_mutex;
+    Buffer jobs_buffer;
+    int jobs_socks[2];
 #endif
 
     /* Encoding specific, if you add something here, don't forget to
-- 
1.7.3.4