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

Chris Johns chrisj at rtems.org
Thu Oct 12 17:35:31 UTC 2017

On 12/10/17 10:05 am, Fan Deng wrote:
> Hi Chris,

Thanks for quick response.

> Based on my understanding, the patch in your email is different:
> 1) rtems_rfs_bitmap_map_set:
> By changing negating the if condition, the updated logic only modifies the
> element map[index] when the original value of map[index] is

Your change has:

 -  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;

The change gets a copy of the uint32_t value from the map and then updates the
map setting the bit. The change then checks to see if the original bit in the
map is set (match returns true) and if set you exit the function. I see this
being the same as the code in my patch where I only proceed to do anything if
the bit is not set in the map:

 -  map[index] = rtems_rfs_bitmap_set (map[index], 1 << offset);
 -  if (rtems_rfs_bitmap_match(map[index], RTEMS_RFS_BITMAP_ELEMENT_SET))
 +  if (!rtems_rfs_bitmap_match(map[index], RTEMS_RFS_BITMAP_ELEMENT_SET))

If the bit is not set you enter the code to handle a map bit being set. You have:

 +  control->free--;
 +  rtems_rfs_buffer_mark_dirty (control->buffer);
    if (rtems_rfs_bitmap_match(map[index], RTEMS_RFS_BITMAP_ELEMENT_SET))

Given you have set the `map[index]` earlier the if test will always pass and I
suspect the compiler will know this and dead code eliminate it.

> This is not quite right, as we are explicitly updating the 'offset' bit in
> map[index]. That bit should be updated regardless.

Why update the bit if it is already set? I seem to be missing something here.
The map is just an array of 32bit unsigned ints.

> 2) rtems_rfs_bitmap_map_clear:
> Same problem, whether we modify map[index] should not depend on if map[index] ==

Sure, once we agree on the set the clear should fall out of it.


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