[Xen-devel] [PATCH] libxl: make libxl_wait_for_device_model not racy

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Hi all,
at the moment libxl_wait_for_device_model waits on a xenstore watch
before checking the current value of the xenstore node, that might
contain already the value the function was looking for.
This patch changes libxl_wait_for_device_model so that it checks the
value of the xenstore node first, then waits for the watch.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

---

diff -r e2f5e4f3481c tools/libxl/libxl_device.c
--- a/tools/libxl/libxl_device.c        Tue Jun 22 16:22:30 2010 +0100
+++ b/tools/libxl/libxl_device.c        Wed Jun 23 11:04:49 2010 +0100
@@ -418,7 +418,7 @@
     char *path;
     char *p;
     unsigned int len;
-    int rc;
+    int rc = 0;
     struct xs_handle *xsh;
     int nfds;
     fd_set rfds;
@@ -432,28 +432,29 @@
     tv.tv_sec = LIBXL_DEVICE_MODEL_START_TIMEOUT;
     tv.tv_usec = 0;
     nfds = xs_fileno(xsh) + 1;
-    while (tv.tv_sec > 0) {
+    while (rc > 0 || (!rc && tv.tv_sec > 0)) {
+        p = xs_read(xsh, XBT_NULL, path, &len);
+        if (p && (!state || !strcmp(state, p))) {
+            free(p);
+            xs_unwatch(xsh, path, path);
+            xs_daemon_close(xsh);
+            if (check_callback) {
+                rc = check_callback(ctx, check_callback_userdata);
+                if (rc) return rc;
+            }
+            return 0;
+        }
+        free(p);
+again:
         FD_ZERO(&rfds);
         FD_SET(xs_fileno(xsh), &rfds);
-        if (select(nfds, &rfds, NULL, NULL, &tv) > 0) {
+        rc = select(nfds, &rfds, NULL, NULL, &tv);
+        if (rc > 0) {
             l = xs_read_watch(xsh, &num);
-            if (l != NULL) {
+            if (l != NULL)
                 free(l);
-                p = xs_read(xsh, XBT_NULL, path, &len);
-                if (!p)
-                    continue;
-                if (!state || !strcmp(state, p)) {
-                    free(p);
-                    xs_unwatch(xsh, path, path);
-                    xs_daemon_close(xsh);
-                    if (check_callback) {
-                        rc = check_callback(ctx, check_callback_userdata);
-                        if (rc) return rc;
-                    }
-                    return 0;
-                }
-                free(p);
-            }
+            else
+                goto again;
         }
     }
     xs_unwatch(xsh, path, path);

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