Also added qemu_logfile_init() for initializing the logfile mutex.

Note that inside qemu_set_log() we needed to add a pair of
qemu_mutex_unlock() calls in order to avoid a double lock in
qemu_log_close().  This unavoidable temporary ugliness will be
cleaned up in a later patch in this series.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
---
v3
    - Removed assert that mutex is initialized.  
      We will rely on the constructor.
    - Also added details in the commit message regarding the 
      temporary ugliness that will be cleaned up in a later patch.
---
v2
    - In qemu_set_log() moved location of mutex lock/unlock 
      due to cleanup changes.
---
v1
    - changed qemu_logfile_init() to use __constructor__.
---
 util/log.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/util/log.c b/util/log.c
index 417d16ec66..953a66b5a8 100644
--- a/util/log.c
+++ b/util/log.c
@@ -24,8 +24,10 @@
 #include "qapi/error.h"
 #include "qemu/cutils.h"
 #include "trace/control.h"
+#include "qemu/thread.h"
 
 static char *logfilename;
+static QemuMutex qemu_logfile_mutex;
 FILE *qemu_logfile;
 int qemu_loglevel;
 static int log_append = 0;
@@ -49,6 +51,11 @@ int qemu_log(const char *fmt, ...)
     return ret;
 }
 
+static void __attribute__((__constructor__)) qemu_logfile_init(void)
+{
+    qemu_mutex_init(&qemu_logfile_mutex);
+}
+
 static bool log_uses_own_buffers;
 
 /* enable or disable low levels log */
@@ -70,7 +77,9 @@ void qemu_set_log(int log_flags)
     if (qemu_loglevel && (!is_daemonized() || logfilename)) {
         need_to_open_file = true;
     }
+    qemu_mutex_lock(&qemu_logfile_mutex);
     if (qemu_logfile && !need_to_open_file) {
+        qemu_mutex_unlock(&qemu_logfile_mutex);
         qemu_log_close();
     } else if (!qemu_logfile && need_to_open_file) {
         if (logfilename) {
@@ -105,6 +114,7 @@ void qemu_set_log(int log_flags)
 #endif
             log_append = 1;
         }
+        qemu_mutex_unlock(&qemu_logfile_mutex);
     }
 }
 
@@ -240,12 +250,14 @@ void qemu_log_flush(void)
 /* Close the log file */
 void qemu_log_close(void)
 {
+    qemu_mutex_lock(&qemu_logfile_mutex);
     if (qemu_logfile) {
         if (qemu_logfile != stderr) {
             fclose(qemu_logfile);
         }
         qemu_logfile = NULL;
     }
+    qemu_mutex_unlock(&qemu_logfile_mutex);
 }
 
 const QEMULogItem qemu_log_items[] = {
-- 
2.17.1