[PATCH 4.19 342/346] xen/xenbus: Count pending messages for each watch

Greg Kroah-Hartman
2020-12-28T13:36:31Z

From: SeongJae Park <sjpark@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: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@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.