Nasty md/raid bug in 3.2.{14,15,16} and 3.3.{1,2,3}

The following is a copy of http://neil.brown.name/blog/20120615073245
which I posted a link to under a less obvious subject, so I'm posting
it again complete with a better subject to maximise the number of people
who will see it.

NeilBrown

A Nasty md/raid bug
=-=-=-=-=-=-=-=-=-=

There is a rather nasty RAID bug in some released versions of the
Linux kernel.  It won't destroy your data, but it could make it hard
to access that data.

If you are concerned that this might affect you, the first thing you
should do (after not panicking) is to gather the output of

   mdadm -Evvvvs

and save this somewhere that is not on a RAID array.  The second thing
to do is read to the end of this note and then proceed accordingly.
You most likely will never need use the output of that command, but if you
do it could be extremely helpful.

Who is vulnerable
=================

The bug was introduced by