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    SubjectRe: [PATCH -v3 5/8] fsnotify: unified filesystem notification backend
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    On Fri, 2008-11-28 at 18:22 -0500, Eric Paris wrote:
    > On Thu, 2008-11-27 at 17:20 +0100, Peter Zijlstra wrote:
    > > On Tue, 2008-11-25 at 12:21 -0500, Eric Paris wrote:
    > > > +int fsnotify_check_notif_queue(struct fsnotify_group *group)
    > > > +{
    > > > + mutex_lock(&group->notification_mutex);
    > > > + if (!list_empty(&group->notification_list))
    > > > + return 1;
    > > > + mutex_unlock(&group->notification_mutex);
    > > > + return 0;
    > > > +}
    > >
    > > > +void fsnotify_clear_notif(struct fsnotify_group *group)
    > > > +{
    > > > + struct fsnotify_event *event;
    > > > +
    > > > + while (fsnotify_check_notif_queue(group)) {
    > > > + event = get_event_from_notif(group);
    > > > + fsnotify_put_event(event);
    > > > + /* fsnotify_check_notif_queue() took this lock */
    > > > + mutex_unlock(&group->notification_mutex);
    > > > + }
    > > > +}
    > >
    > > That is quite horrible, please just open code that to keep the locking
    > > symmetric.
    >
    > While horrible, I use fsnotify_check_notif_queue in my fsnotify (not in
    > this series as this only includes dnotify) has
    >
    > wait_event_interruptible(group->notification_waitq, fanotify_check_notif_queue(group));
    >
    > So I wouldn't know how to open code that... I can open code this
    > instance, but it's going to mean redoing all of that other code to
    > handle having thing not be present when we return. Since I didn't
    > submit that as well I guess I'm not allowed to use it as a reason...

    Or you add a lock parameter to wait_event*() which gets unlocked before
    schedule and locks again afterwards.

    That would allow you to write it like so:

    mutex_lock(&group->notification_mutex);
    wait_event_interruptible_lock(group->notification_waitq,
    !list_empty(&group_notificatioin_list),
    &group_notification_mutex);

    /* handle the !empty list */
    mutex_unlock(&group->notification_mutex);

    You could use the type matching magic we have to select between
    spinlock/mutex operations for the lock argument.

    I've come across such a pattern a few times, most of the times we end up
    open coding the wait_event stuff.



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