monitor: maintain at most one G_IO_OUT watch

When monitor_flush() is invoked repeatedly outside the monitor_unblocked()
callback, for example from tlb_info() -> ... -> print_pte(), several
watches may be added for the same event.

This is no problem per se because the extra monitor_unblocked() callbacks
are harmless if mon->outbuf is empty, the watches will be removed
gradually. However a big number of watches can grow "gpollfds" without
limit in glib_pollfds_fill(), triggering a -1/EINVAL condition in
g_poll().

Keep at most one such watch, by following the pattern observable in eg.
commits c874ea97 and c3d6b96e. The change has no effect when
monitor_unblocked() calls monitor_flush() (when the watch can either be
removed or renewed 1-for-1), but non-callback contexts won't create an
additional watch when the monitor already has one.

Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=970047

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1373998781-29561-3-git-send-email-lersek@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/monitor.c b/monitor.c
index 99b6ed9060..6db2ba4d7a 100644
--- a/monitor.c
+++ b/monitor.c
@@ -189,6 +189,7 @@ struct Monitor {
     int suspend_cnt;
     bool skip_flush;
     QString *outbuf;
+    guint watch;
     ReadLineState *rs;
     MonitorControl *mc;
     CPUState *mon_cpu;
@@ -263,7 +264,10 @@ int monitor_read_password(Monitor *mon, ReadLineFunc *readline_func,
 static gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
                                   void *opaque)
 {
-    monitor_flush(opaque);
+    Monitor *mon = opaque;
+
+    mon->watch = 0;
+    monitor_flush(mon);
     return FALSE;
 }
 
@@ -294,7 +298,10 @@ void monitor_flush(Monitor *mon)
             QDECREF(mon->outbuf);
             mon->outbuf = tmp;
         }
-        qemu_chr_fe_add_watch(mon->chr, G_IO_OUT, monitor_unblocked, mon);
+        if (mon->watch == 0) {
+            mon->watch = qemu_chr_fe_add_watch(mon->chr, G_IO_OUT,
+                                               monitor_unblocked, mon);
+        }
     }
 }