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    Subject[PATCH 5.10 699/717] xen/xenbus: Count pending messages for each watch
    Date
    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
    @@ -61,6 +61,8 @@ 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.

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