[Qemu-devel] [PATCH 04/16] qemu-char: Fix a race reporting opens and closes

[minyard@acm.org]

From: Corey Minyard <cminyard@mvista.com>

When an open event is reported, it is done through a bh.  But close
events are reported immediately.  So if an open event is in the bh
and a close occurs, an extraneous open happens, which can confuse a
user.

To fix this, this patch sets the "opened" flag immediately instead
of in the bh handler and checks to make sure the opened flag is
set before reporting an open event.

This also modifies the spice code to call qemu_chr_generic_open
to report an open, to keep things consistent.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
---
 qemu-char.c       |   18 ++++++++++++++++--
 spice-qemu-char.c |    7 +++++--
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/qemu-char.c b/qemu-char.c
index f6a671b..9e3ac40 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -111,10 +111,13 @@ void qemu_chr_be_event(CharDriverState *s, int event)
     /* Keep track if the char device is open */
     switch (event) {
         case CHR_EVENT_OPENED:
+            /*
+             * See the comment in qemu_chr_generic_open_bh() for why
+             * 's->opened = 1' is not here.
+             */
             if (s->recon_timer) {
                 qemu_del_timer(s->recon_timer);
             }
-            s->opened = 1;
             break;
         case CHR_EVENT_CLOSED:
             if (s->recon_timer) {
@@ -134,13 +137,24 @@ void qemu_chr_be_event(CharDriverState *s, int event)
 static void qemu_chr_generic_open_bh(void *opaque)
 {
     CharDriverState *s = opaque;
-    qemu_chr_be_event(s, CHR_EVENT_OPENED);
+    /*
+     * Since the "close" event doesn't go through a bh, there is a
+     * possible race condition if a close comes in after an open, but
+     * the open is in the bh queue.  So we double-check here, and we
+     * set opened in qemu_chr_generic_open() instead of
+     * qemu_chr_be_event().
+     */
+    if (s->opened) {
+        qemu_chr_be_event(s, CHR_EVENT_OPENED);
+    }
     qemu_bh_delete(s->bh);
     s->bh = NULL;
 }
 
 void qemu_chr_generic_open(CharDriverState *s)
 {
+    /* See the comment in qemu_chr_generic_open_bh() for why this is here */
+    s->opened = 1;
     if (s->bh == NULL) {
 	s->bh = qemu_bh_new(qemu_chr_generic_open_bh, s);
 	qemu_bh_schedule(s->bh);
diff --git a/spice-qemu-char.c b/spice-qemu-char.c
index 2fb8a10..d7516bd 100644
--- a/spice-qemu-char.c
+++ b/spice-qemu-char.c
@@ -93,8 +93,11 @@ static void vmc_state(SpiceCharDeviceInstance *sin, int connected)
         return;
     }
 
-    qemu_chr_be_event(scd->chr,
-                      connected ? CHR_EVENT_OPENED : CHR_EVENT_CLOSED);
+    if (connected) {
+        qemu_chr_generic_open(scd->chr);
+    } else {
+        qemu_chr_be_event(scd->chr, CHR_EVENT_CLOSED);
+    }
 }
 
 static SpiceCharDeviceInterface vmc_interface = {
-- 
1.7.4.1