[Crash-utility] [PATCH] Support cpu_map/cpu_mask changes in 2.6.29

[Dave Anderson]

Michael,

It's not clear to me how this function works:

+ * Get cpu map (possible, online, etc.) address from cpu mask (since 2.6.29)
+ */
+static ulong
+get_cpu_map_addr_from_mask(const char *type)
+{
+       ulong cpu_mask_addr, cpu_map_addr;
+       char cpu_mask_symbol[32];
+       char *cpu_mask_buf;
+       int cpu_mask_size;
+
+       sprintf(cpu_mask_symbol, "cpu_%s_mask", type);
+
+       if (!symbol_exists(cpu_mask_symbol))
+               return 0;
+
+       cpu_mask_addr = symbol_value(cpu_mask_symbol);
+       cpu_mask_size = STRUCT_SIZE("cpumask");
+       cpu_mask_buf = malloc(cpu_mask_size);
+       if (!cpu_mask_buf)
+               error(FATAL, "get_cpu_map_addr_from_mask: out of memory\n");
+       readmem(cpu_mask_addr, KVADDR, cpu_mask_buf, cpu_mask_size,
+               cpu_mask_symbol, FAULT_ON_ERROR);
+       cpu_map_addr = ULONG(cpu_mask_buf + MEMBER_OFFSET("cpumask", "bits"));
+       free(cpu_mask_buf);
+
+       return cpu_map_addr;
+}

The caller to this function expects the address of a cpumask_t
data structure.  As I understand it, both before and after 2.6.29,
the cpumask data structure looks like:

  typedef struct cpumask {
      unsigned long int bits[<variable-based-upon-NR_CPUS>];
  } cpumask_t;

And pre-2.6.29, the 3 "cpu_xxxx_map" symbols were all statically
defined cpumask structures:

  cpumask_t cpu_online_mask;
  cpumask_t cpu_possible_mask;
  cpumask_t cpu_present_mask;

Whereas now the 3 symbols don't exist except as #define's like so:

  #define cpu_possible_map        (*(cpumask_t *)cpu_possible_mask)
  #define cpu_online_map          (*(cpumask_t *)cpu_online_mask)
  #define cpu_present_map         (*(cpumask_t *)cpu_present_mask)

and -- if I understand the code correctly -- each cpu_xxxx_mask 
is just a pointer to a cpumask data structure.

So when the caller needs the address of the relevant cpumask data structure
why doesn't your cpu_map_addr_from_mask() function simply do something
like:

  get_symbol_data(cpu_mask_symbol, sizeof(void *), &tmp);
  return tmp;

i.e., which would read the contents of the cpu_mask_symbol, which
is the pointer that the caller wants.
  
Instead, your function reads in the whole data structure, where
the cpumask.bits[] array typically contains several items, i.e.,
enough longs to contain NR_CPUS bits.  Then, with ULONG(), it is
essentially reading the unsigned long value found in the first
bit[] array item, or:
 
   cpumask.bits[0];

and therefore returning the *bitmask* contained in the first unsigned
long in the bit[] array -- instead of the *address* of the cpumask
data structure.  How can that work? 

Or am I missing something?

(Sorry -- I just started looking at this again, but I don't have
a test kernel of this vintage to play with...)

Dave