[PATCH v3 4/5] xen/xenbus: Count pending messages for each watch

[SeongJae Park]

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

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

This is upstream commit 3dc86ca6b4c8cfcba9da7996189d1b5a358a94fc

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>
---
 drivers/xen/xenbus/xenbus_xs.c | 31 +++++++++++++++++++------------
 include/xen/xenbus.h           |  2 ++
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index 0ea1c259f2f1..d98d88fae58a 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -701,6 +701,8 @@ int register_xenbus_watch(struct xenbus_watch *watch)
 
        sprintf(token, "%lX", (long)watch);
 
+       watch->nr_pending = 0;
+
        down_read(&xs_state.watch_mutex);
 
        spin_lock(&watches_lock);
@@ -750,12 +752,15 @@ void unregister_xenbus_watch(struct xenbus_watch *watch)
 
        /* Cancel pending watch events. */
        spin_lock(&watch_events_lock);
-       list_for_each_entry_safe(msg, tmp, &watch_events, list) {
-               if (msg->u.watch.handle != watch)
-                       continue;
-               list_del(&msg->list);
-               kfree(msg->u.watch.vec);
-               kfree(msg);
+       if (watch->nr_pending) {
+               list_for_each_entry_safe(msg, tmp, &watch_events, list) {
+                       if (msg->u.watch.handle != watch)
+                               continue;
+                       list_del(&msg->list);
+                       kfree(msg->u.watch.vec);
+                       kfree(msg);
+               }
+               watch->nr_pending = 0;
        }
        spin_unlock(&watch_events_lock);
 
@@ -802,7 +807,6 @@ void xs_suspend_cancel(void)
 
 static int xenwatch_thread(void *unused)
 {
-       struct list_head *ent;
        struct xs_stored_msg *msg;
 
        for (;;) {
@@ -815,13 +819,15 @@ 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);
+               msg = list_first_entry_or_null(&watch_events,
+                               struct xs_stored_msg, list);
+               if (msg) {
+                       list_del(&msg->list);
+                       msg->u.watch.handle->nr_pending--;
+               }
                spin_unlock(&watch_events_lock);
 
-               if (ent != &watch_events) {
-                       msg = list_entry(ent, struct xs_stored_msg, list);
+               if (msg) {
                        msg->u.watch.handle->callback(
                                msg->u.watch.handle,
                                (const char **)msg->u.watch.vec,
@@ -911,6 +917,7 @@ static int process_msg(void)
                                         msg->u.watch.vec_size))) {
                        spin_lock(&watch_events_lock);
                        list_add_tail(&msg->list, &watch_events);
+                       msg->u.watch.handle->nr_pending++;
                        wake_up(&watch_events_waitq);
                        spin_unlock(&watch_events_lock);
                } else {
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 1772507dc2c9..ed9e7e3307b7 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -58,6 +58,8 @@ 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.
-- 
2.17.1