Re: [PATCH 2/2] imsm: FIX: Be more patient during loading matadata

[Dan Williams <dan.j.williams@intel.com>]

On 4/14/2011 1:08 AM, NeilBrown wrote:
>> Why does this existing check:
>>
>>                 /* retry the load if we might have raced against mdmon */
>>                  if (err == 3&&  mdmon_running(devnum))
>>                          for (retry = 0; retry<  3; retry++) {
>>                                  usleep(3000);
>>                                  err = load_and_parse_mpb(dfd, s, NULL, 1);
>>                                  if (err != 3)
>>                                          break;
>>                          }
>
> It's pretty horrible that we need to do this, isn't it?

Yes.

> Maybe we need some way to synchronise with mdmon so we *know* if we have
> raced or not.
> i.e. mdmon keeps a count of the number of times it has updated the metadata.
> We send a message to get the count, read, then get the count again.
> The request blocks while mdmon is actually writing.
> If the counts are different, we read again.
> ??

Assuming we are only racing against dirty transitions maybe it is enough 
to poll md/array_state and if it signaled while reading the metadata we 
likely need to try again (of course with an intervening ping_monitor)?

--
Dan