[gegl] buffer: gegl_tile_lock do context switches when tile is locked

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 9f134d4e14b8d3108de61972f2a91ba3863d3539
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Apr 28 21:23:29 2020 +0200

    buffer: gegl_tile_lock do context switches when tile is locked
    
    On hyper threading systems the number of active threads can be higher
    than the number of actual processing cores, by adding sleeps between
    attempts after a limit, we avoid some starvation that happens in these
    cases. Patch from @suzu in gimp#4955.

 gegl/buffer/gegl-tile.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gegl/buffer/gegl-tile.c b/gegl/buffer/gegl-tile.c
index 353d3dbc9..68f239eed 100644
--- a/gegl/buffer/gegl-tile.c
+++ b/gegl/buffer/gegl-tile.c
@@ -296,6 +296,7 @@ end:
 void
 gegl_tile_lock (GeglTile *tile)
 {
+  unsigned int count = 0;
   g_atomic_int_inc (&tile->lock_count);
 
   while (TRUE)
@@ -321,6 +322,10 @@ gegl_tile_lock (GeglTile *tile)
         case CLONE_STATE_UNCLONING:
           break;
         }
+
+      ++count;
+      if (count > 32)
+        usleep (1);
     }
 }