Re: [RFC PATCH -v4 12/14] fsnotify: add correlations between events

["C. Scott Ananian" <cscott@laptop.org>]

On Fri, Dec 12, 2008 at 4:52 PM, Eric Paris <eparis@redhat.com> wrote:
> inotify sends userspace a correlation between events when they are related
> (aka when dentries are moved).  This adds that same support for all
> fsnotify events.
> diff --git a/fs/notify/notification.c b/fs/notify/notification.c
> index 8ed9d32..7243b20 100644
> --- a/fs/notify/notification.c
> +++ b/fs/notify/notification.c
> @@ -34,6 +35,13 @@
>
>  static struct kmem_cache *event_kmem_cache;
>  static struct kmem_cache *event_holder_kmem_cache;
> +static atomic_t fsnotify_sync_cookie = ATOMIC_INIT(0);
> +
> +u32 fsnotify_get_cookie(void)
> +{
> +        return atomic_inc_return(&fsnotify_sync_cookie);
> +}
> +EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
>
>  int fsnotify_check_notif_queue(struct fsnotify_group *group)
>  {

atomic_inc_return seems rather expensive to put on a hot path in
almost every fs operation.  On a multiprocessor system, the cache line
for fsnotify_sync_cookie would be ping-ponging constantly between
processors.  The canonical solution is to form the cookie by
concatenating the processor number with a per-processor cookie, so
that generating a new cookie would not require synchronization between
processors.  Surely this code already exists to be used somewhere in
Linux.
 --scott

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