[PATCH] Fixes bitmap allocation accounting logic in rtems-rfs-bitmaps.c

[Fan Deng]

The bitmap allocation accounting logic in rtems-rfs-bitmaps.c is flawed
around control->free. Specifically:

In rtems_rfs_bitmap_map_set():
control->free is only decremented when its corresponding search bit is
toggled. This is wrong and will miss on average 31/32 set updates.

In rtems_rfs_bitmap_map_clear():
control->free is incremented unconditionally.

The correct behavior is:
When updating the map, check if the bit is already set/clear. Only update
control->free when the bit is toggled.

This change enforced the correct behavior.

Tested by inspecting the internal data structure.
---
 cpukit/libfs/src/rfs/rtems-rfs-bitmaps.c | 40 +++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/cpukit/libfs/src/rfs/rtems-rfs-bitmaps.c b/cpukit/libfs/src/rfs/rtems-rfs-bitmaps.c
index 15a9050133..c0c6921db1 100644
--- a/cpukit/libfs/src/rfs/rtems-rfs-bitmaps.c
+++ b/cpukit/libfs/src/rfs/rtems-rfs-bitmaps.c
@@ -183,11 +183,12 @@ int
 rtems_rfs_bitmap_map_set (rtems_rfs_bitmap_control* control,
                           rtems_rfs_bitmap_bit      bit)
 {
-  rtems_rfs_bitmap_map map;
-  rtems_rfs_bitmap_map search_map;
-  int                  index;
-  int                  offset;
-  int                 rc;
+  rtems_rfs_bitmap_map     map;
+  rtems_rfs_bitmap_map     search_map;
+  int                      index;
+  int                      offset;
+  int                      rc;
+  rtems_rfs_bitmap_element element;
   rc = rtems_rfs_bitmap_load_map (control, &map);
   if (rc > 0)
     return rc;
@@ -196,15 +197,20 @@ rtems_rfs_bitmap_map_set (rtems_rfs_bitmap_control* control,
   search_map = control->search_bits;
   index      = rtems_rfs_bitmap_map_index (bit);
   offset     = rtems_rfs_bitmap_map_offset (bit);
-  map[index] = rtems_rfs_bitmap_set (map[index], 1 << offset);
+  element    = map[index];
+  map[index] = rtems_rfs_bitmap_set (element, 1 << offset);
+  // If the element does not change, the bit was already set. There will be no
+  // further action to take.
+  if (rtems_rfs_bitmap_match(element, map[index]))
+      return 0;
+  control->free--;
+  rtems_rfs_buffer_mark_dirty (control->buffer);
   if (rtems_rfs_bitmap_match(map[index], RTEMS_RFS_BITMAP_ELEMENT_SET))
   {
     bit = index;
     index  = rtems_rfs_bitmap_map_index (bit);
     offset = rtems_rfs_bitmap_map_offset (bit);
     search_map[index] = rtems_rfs_bitmap_set (search_map[index], 1 << offset);
-    control->free--;
-    rtems_rfs_buffer_mark_dirty (control->buffer);
   }
   return 0;
 }
@@ -213,11 +219,12 @@ int
 rtems_rfs_bitmap_map_clear (rtems_rfs_bitmap_control* control,
                             rtems_rfs_bitmap_bit      bit)
 {
-  rtems_rfs_bitmap_map map;
-  rtems_rfs_bitmap_map search_map;
-  int                  index;
-  int                  offset;
-  int                  rc;
+  rtems_rfs_bitmap_map      map;
+  rtems_rfs_bitmap_map     search_map;
+  int                      index;
+  int                      offset;
+  int                      rc;
+  rtems_rfs_bitmap_element element;
   rc = rtems_rfs_bitmap_load_map (control, &map);
   if (rc > 0)
     return rc;
@@ -226,7 +233,12 @@ rtems_rfs_bitmap_map_clear (rtems_rfs_bitmap_control* control,
   search_map        = control->search_bits;
   index             = rtems_rfs_bitmap_map_index (bit);
   offset            = rtems_rfs_bitmap_map_offset (bit);
-  map[index]        = rtems_rfs_bitmap_clear (map[index], 1 << offset);
+  element           = map[index];
+  map[index]        = rtems_rfs_bitmap_clear (element, 1 << offset);
+  // If the element does not change, the bit was already clear. There will be no
+  // further action to take.
+  if (rtems_rfs_bitmap_match(element, map[index]))
+      return 0;
   bit               = index;
   index             = rtems_rfs_bitmap_map_index (bit);
   offset            = rtems_rfs_bitmap_map_offset(bit);
-- 
2.14.2.920.gcf0c67979c-goog