[PATCH 5.4 446/453] xen/xenbus: Count pending messages for each watch

[Greg Kroah-Hartman]

From: SeongJae Park <sjp...@amazon.de>

commit 3dc86ca6b4c8cfcba9da7996189d1b5a358a94fc upstream.

This commit adds a counter of pending messages for each watch in the
struct.  It is used to skip unnecessary pending messages lookup in
'unregister_xenbus_watch()'.  It could also be used in 'will_handle'
callback.

This is part of XSA-349

Cc: sta...@vger.kernel.org
Signed-off-by: SeongJae Park <sjp...@amazon.de>
Reported-by: Michael Kurth <m...@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipa...@amazon.de>
Reviewed-by: Juergen Gross <jgr...@suse.com>
Signed-off-by: Juergen Gross <jgr...@suse.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/xen/xenbus/xenbus_xs.c |   29 ++++++++++++++++++-----------
 include/xen/xenbus.h           |    2 ++
 2 files changed, 20 insertions(+), 11 deletions(-)

--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -711,6 +711,7 @@ int xs_watch_msg(struct xs_watch_event *
                                 event->path, event->token))) {
                spin_lock(&watch_events_lock);
                list_add_tail(&event->list, &watch_events);
+               event->handle->nr_pending++;
                wake_up(&watch_events_waitq);
                spin_unlock(&watch_events_lock);
        } else
@@ -768,6 +769,8 @@ int register_xenbus_watch(struct xenbus_
 
        sprintf(token, "%lX", (long)watch);
 
+       watch->nr_pending = 0;
+
        down_read(&xs_watch_rwsem);
 
        spin_lock(&watches_lock);
@@ -817,11 +820,14 @@ void unregister_xenbus_watch(struct xenb
 
        /* Cancel pending watch events. */
        spin_lock(&watch_events_lock);
-       list_for_each_entry_safe(event, tmp, &watch_events, list) {
-               if (event->handle != watch)
-                       continue;
-               list_del(&event->list);
-               kfree(event);
+       if (watch->nr_pending) {
+               list_for_each_entry_safe(event, tmp, &watch_events, list) {
+                       if (event->handle != watch)
+                               continue;
+                       list_del(&event->list);
+                       kfree(event);
+               }
+               watch->nr_pending = 0;
        }
        spin_unlock(&watch_events_lock);
 
@@ -868,7 +874,6 @@ void xs_suspend_cancel(void)
 
 static int xenwatch_thread(void *unused)
 {
-       struct list_head *ent;
        struct xs_watch_event *event;
 
        xenwatch_pid = current->pid;
@@ -883,13 +888,15 @@ static int xenwatch_thread(void *unused)
                mutex_lock(&xenwatch_mutex);
 
                spin_lock(&watch_events_lock);
-               ent = watch_events.next;
-               if (ent != &watch_events)
-                       list_del(ent);
+               event = list_first_entry_or_null(&watch_events,
+                               struct xs_watch_event, list);
+               if (event) {
+                       list_del(&event->list);
+                       event->handle->nr_pending--;
+               }
                spin_unlock(&watch_events_lock);
 
-               if (ent != &watch_events) {
-                       event = list_entry(ent, struct xs_watch_event, list);
+               if (event) {
                        event->handle->callback(event->handle, event->path,
                                                event->token);
                        kfree(event);
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -59,6 +59,8 @@ struct xenbus_watch
        /* Path being watched. */
        const char *node;
 
+       unsigned int nr_pending;
+
        /*
         * Called just before enqueing new event while a spinlock is held.
         * The event will be discarded if this callback returns false.