[Qemu-devel] [PATCH] monitor: Protect outbuf from concurrent access

[Luiz Capitulino]

Sometimes, when having lots of VMs running on a RHEV host and the user
attempts to close a SPICE window, libvirt will get corrupted json from
QEMU.

After some investigation, I found out that the problem is that different
SPICE threads are calling monitor functions (such as
monitor_protocol_event()) in parallel which causes concurrent access
to the monitor's internal buffer outbuf[].

This fixes the problem by protecting accesses to outbuf[] with a mutex.

Honestly speaking, I'm not completely sure this the best thing to do
because the monitor itself and other qemu subsystems are not thread safe,
so having subsystems like SPICE assuming the contrary seems a bit
catastrophic to me...

Anyways, this commit fixes the problem at hand.

Signed-off-by: Luiz Capitulino <address@hidden>
---
 monitor.c |   16 +++++++++++++++-
 1 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/monitor.c b/monitor.c
index 04f465a..61d4d93 100644
--- a/monitor.c
+++ b/monitor.c
@@ -57,6 +57,7 @@
 #include "json-parser.h"
 #include "osdep.h"
 #include "cpu.h"
+#include "qemu-thread.h"
 #ifdef CONFIG_SIMPLE_TRACE
 #include "trace.h"
 #endif
@@ -144,6 +145,7 @@ struct Monitor {
     int suspend_cnt;
     uint8_t outbuf[1024];
     int outbuf_index;
+    QemuMutex mutex;
     ReadLineState *rs;
     MonitorControl *mc;
     CPUState *mon_cpu;
@@ -246,10 +248,14 @@ static int monitor_read_password(Monitor *mon, 
ReadLineFunc *readline_func,
 
 void monitor_flush(Monitor *mon)
 {
+    qemu_mutex_lock(&mon->mutex);
+
     if (mon && mon->outbuf_index != 0 && !mon->mux_out) {
         qemu_chr_fe_write(mon->chr, mon->outbuf, mon->outbuf_index);
         mon->outbuf_index = 0;
     }
+
+    qemu_mutex_unlock(&mon->mutex);
 }
 
 /* flush at every end of line or if the buffer is full */
@@ -257,6 +263,8 @@ static void monitor_puts(Monitor *mon, const char *str)
 {
     char c;
 
+    qemu_mutex_lock(&mon->mutex);
+
     for(;;) {
         c = *str++;
         if (c == '\0')
@@ -265,9 +273,14 @@ static void monitor_puts(Monitor *mon, const char *str)
             mon->outbuf[mon->outbuf_index++] = '\r';
         mon->outbuf[mon->outbuf_index++] = c;
         if (mon->outbuf_index >= (sizeof(mon->outbuf) - 1)
-            || c == '\n')
+            || c == '\n') {
+            qemu_mutex_unlock(&mon->mutex);
             monitor_flush(mon);
+            qemu_mutex_lock(&mon->mutex);
+        }
     }
+
+    qemu_mutex_unlock(&mon->mutex);
 }
 
 void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
@@ -5275,6 +5288,7 @@ void monitor_init(CharDriverState *chr, int flags)
 
     mon = g_malloc0(sizeof(*mon));
 
+    qemu_mutex_init(&mon->mutex);
     mon->chr = chr;
     mon->flags = flags;
     if (flags & MONITOR_USE_READLINE) {
-- 
1.7.7.rc0.72.g4b5ea