Improve suspend eventchn lock, use flock instead of exclusive lock
file to avoid that sometimes the lock file is not removed cleanly 
and affacts later getting lock.
http://lists.xensource.com/archives/html/xen-devel/2010-11/msg01559.html 

Signed-off-by cyliu@novell.com

Index: xen-4.0.1-testing/tools/libxc/xc_suspend.c
===================================================================
--- xen-4.0.1-testing.orig/tools/libxc/xc_suspend.c
+++ xen-4.0.1-testing/tools/libxc/xc_suspend.c
@@ -6,25 +6,25 @@
 
 #include "xc_private.h"
 #include "xenguest.h"
+#include <sys/file.h>
+#include <sys/fcntl.h>
 
 #define SUSPEND_LOCK_FILE "/var/lib/xen/suspend_evtchn_lock.d"
 static int lock_suspend_event(void)
 {
     int fd, rc;
     mode_t mask;
-    char buf[128];
 
     mask = umask(022);
-    fd = open(SUSPEND_LOCK_FILE, O_CREAT | O_EXCL | O_RDWR, 0666);
+    fd = open(SUSPEND_LOCK_FILE, O_CREAT | O_RDWR, 0666);
     if (fd < 0)
     {
         ERROR("Can't create lock file for suspend event channel\n");
         return -EINVAL;
     }
     umask(mask);
-    snprintf(buf, sizeof(buf), "%10ld", (long)getpid());
+    rc = flock(fd, LOCK_EX | LOCK_NB);
 
-    rc = write_exact(fd, buf, strlen(buf));
     close(fd);
 
     return rc;
@@ -32,30 +32,21 @@ static int lock_suspend_event(void)
 
 static int unlock_suspend_event(void)
 {
-    int fd, pid, n;
-    char buf[128];
+    int fd, rc;
 
     fd = open(SUSPEND_LOCK_FILE, O_RDWR);
 
     if (fd < 0)
         return -EINVAL;
 
-    n = read(fd, buf, 127);
+    rc = flock(fd, LOCK_UN | LOCK_NB);
 
     close(fd);
 
-    if (n > 0)
-    {
-        sscanf(buf, "%d", &pid);
-        /* We are the owner, so we can simply delete the file */
-        if (pid == getpid())
-        {
-            unlink(SUSPEND_LOCK_FILE);
-            return 0;
-        }
-    }
+    if(!rc)
+	unlink(SUSPEND_LOCK_FILE);
 
-    return -EPERM;
+    return rc;
 }
 
 int xc_await_suspend(int xce, int suspend_evtchn)
@@ -110,8 +101,7 @@ int xc_suspend_evtchn_init(int xc, int x
     return suspend_evtchn;
 
 cleanup:
-    if (suspend_evtchn != -1)
-        xc_suspend_evtchn_release(xce, suspend_evtchn);
+    xc_suspend_evtchn_release(xce, suspend_evtchn);
 
     return -1;
 }