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    Subject[PATCH v3 4/5] xen/xenbus: Count pending messages for each watch
    Date
    From: SeongJae Park <sjpark@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: 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>
    ---
    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
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