For raid5 on an array with more than 3 drive, if you attempt to write
a single block, it will:

 - read the current value of the block, and the parity block.
 - "subtract" the old value of the block from the parity, and "add"
   the new value.
 - write out the new data and the new parity.

If the parity was wrong before, it will still be wrong.  If you then
lose a drive, you lose your data.

Wow, that really needs to be put somewhere in 120 point red blinking
text.  A lot of us are used to uninitialized disks calculating the
parity-on-first-write, but if linux MD is forgoeing that
'dangerous-no-resync' sounds really REALLY bad.  How about at least a
'Warning: unlike other systems this WILL cause corruption if you
forego reconstruction' on mkraid?