[Spice-devel] [spice-gtk PATCH v2] This adds reference counting to cached images.

[Fabiano Fidêncio]

On Wed, Jun 24, 2015 at 7:09 PM, Sandy Stutsman <sstutsma at redhat.com> wrote:
>
> Windows guests with multi-monitors will often send down the same image
> via different channels.  If these instances are not counted,  one channel
> can delete an image before all the rest of the channels are finished with
> it. In this case, the client will hang.
>
> This can happen, for instance, when the Windows guest is updating the
> monitor's wallpaper. There is a driver instance for each monitor, so each
> one is sending down its own copy of the same image with the same id.  As
> one channel may be busier than another, the order that the client executes
> commands may not be the same order as the server issued them.  Here's what
> can happen:
>
>   On the server side:
>     Monitor 1 sends image A
>     Monitor 1 sends multiple rendering commands for image A
>     Monitor 1 removes image A
>     Monitor 2 sends image A
>     Monitor 2 sends rendering commands for image A
>   On the client side:
>     Monitor 1 adds image A to the cache
>     Monitor 1 starts rendering with image A
>     Monitor 2 adds image A to the cache - has same id, so image is replaced
>     Monitor 1 removes image A - image A is completely removed from cache
>     Monitor 2 render command hangs waiting for image A
>
> Addresses bug: https://bugzilla.redhat.com/show_bug.cgi?id=1194354
> ---
> Change since v1
>   *Add to commit log
> ---
>  src/spice-channel-cache.h | 60 ++++++++++++++++++++++++++++++++++++-----------
>  src/spice-session.c       |  2 +-
>  2 files changed, 47 insertions(+), 15 deletions(-)
>
> diff --git a/src/spice-channel-cache.h b/src/spice-channel-cache.h
> index 17775e6..968e661 100644
> --- a/src/spice-channel-cache.h
> +++ b/src/spice-channel-cache.h
> @@ -27,15 +27,20 @@ G_BEGIN_DECLS
>  typedef struct display_cache_item {
>      guint64                     id;
>      gboolean                    lossy;
> +    uint32_t                    ref_count;

Could be guint32, right?

>  } display_cache_item;
>
> -typedef GHashTable display_cache;
> +typedef struct display_cache {
> +    GHashTable  *table;
> +    gboolean    ref_counted;
> +}display_cache;
>
>  static inline display_cache_item* cache_item_new(guint64 id, gboolean lossy)
>  {
>      display_cache_item *self = g_slice_new(display_cache_item);
>      self->id = id;
>      self->lossy = lossy;
> +    self->ref_count = 1;
>      return self;
>  }
>
> @@ -46,18 +51,24 @@ static inline void cache_item_free(display_cache_item *self)
>
>  static inline display_cache* cache_new(GDestroyNotify value_destroy)
>  {
> -    GHashTable* self;
> -
> -    self = g_hash_table_new_full(g_int64_hash, g_int64_equal,
> -                                 (GDestroyNotify)cache_item_free,
> -                                 value_destroy);
> -
> +    display_cache * self = g_slice_new(display_cache);
> +    self->table = g_hash_table_new_full(g_int64_hash, g_int64_equal,
> +                                       (GDestroyNotify) cache_item_free,
> +                                       value_destroy);
> +    self->ref_counted = FALSE;
>      return self;
>  }
>
> +static inline display_cache * cache_image_new(GDestroyNotify value_destroy)
> +{
> +    display_cache * self = cache_new(value_destroy);
> +    self->ref_counted = TRUE;
> +    return self;
> +};
> +
>  static inline gpointer cache_find(display_cache *cache, uint64_t id)
>  {
> -    return g_hash_table_lookup(cache, &id);
> +    return g_hash_table_lookup(cache->table, &id);
>  }
>
>  static inline gpointer cache_find_lossy(display_cache *cache, uint64_t id, gboolean *lossy)
> @@ -65,7 +76,7 @@ static inline gpointer cache_find_lossy(display_cache *cache, uint64_t id, gbool
>      gpointer value;
>      display_cache_item *item;
>
> -    if (!g_hash_table_lookup_extended(cache, &id, (gpointer*)&item, &value))
> +    if (!g_hash_table_lookup_extended(cache->table, &id, (gpointer*)&item, &value))
>          return NULL;
>
>      *lossy = item->lossy;
> @@ -77,8 +88,17 @@ static inline void cache_add_lossy(display_cache *cache, uint64_t id,
>                                     gpointer value, gboolean lossy)
>  {
>      display_cache_item *item = cache_item_new(id, lossy);
> -
> -    g_hash_table_replace(cache, item, value);
> +    display_cache_item *current_item;
> +    gpointer            current_image;
> +
> +    //If image is currently in the table add its reference count before replacing it
> +    if(cache->ref_counted) {
> +        if(g_hash_table_lookup_extended(cache->table, &id, (gpointer*) &current_item,
> +                                        (gpointer*) &current_image)) {
> +            item->ref_count = current_item->ref_count + 1;
> +        }
> +    }
> +    g_hash_table_replace(cache->table, item, value);
>  }
>
>  static inline void cache_add(display_cache *cache, uint64_t id, gpointer value)
> @@ -88,17 +108,29 @@ static inline void cache_add(display_cache *cache, uint64_t id, gpointer value)
>
>  static inline gboolean cache_remove(display_cache *cache, uint64_t id)
>  {
> -    return g_hash_table_remove(cache, &id);
> +    display_cache_item * item;
> +    gpointer value;
> +
> +    if( g_hash_table_lookup_extended(cache->table, &id, (gpointer*) &item, &value)) {
> +        --item->ref_count;
> +        if(!cache->ref_counted || !item->ref_count ) {

I really would prefer to have comparisons (when not booleans) in the
explicit way, like:
if(!cache->ref_counted || item->ref_count > 0) { ... }

> +            return g_hash_table_remove(cache->table, &id);
> +        }
> +    }
> +    else {
> +        return FALSE;
> +    }
> +    return TRUE;
>  }
>
>  static inline void cache_clear(display_cache *cache)
>  {
> -    g_hash_table_remove_all(cache);
> +    g_hash_table_remove_all(cache->table);
>  }
>
>  static inline void cache_unref(display_cache *cache)
>  {
> -    g_hash_table_unref(cache);
> +    g_hash_table_unref(cache->table);
>  }
>
>  G_END_DECLS
> diff --git a/src/spice-session.c b/src/spice-session.c
> index 778d82a..a252448 100644
> --- a/src/spice-session.c
> +++ b/src/spice-session.c
> @@ -257,7 +257,7 @@ static void spice_session_init(SpiceSession *session)
>      g_free(channels);
>
>      ring_init(&s->channels);
> -    s->images = cache_new((GDestroyNotify)pixman_image_unref);
> +    s->images = cache_image_new((GDestroyNotify)pixman_image_unref);
>      s->glz_window = glz_decoder_window_new();
>      update_proxy(session, NULL);
>  }
> --
> 1.9.5.msysgit.0
>
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Apart from the minors comments, that can or cannot be addressed (up to
Sandy), the patch looks good to me.
-- 
Fabiano Fidêncio