On Fri, Feb 22 2019 at  9:02pm -0500,
John Dorminy [off-list ref] wrote:

I am perhaps not understanding the intricacies here, or not seeing a
barrier protecting it, so forgive me if I'm off base. I think reading
parent->bi_status here is unsafe.
Consider the following sequence of events on two threads.

Thread 0                                 Thread 1
In __bio_chain_endio:                    In __bio_chain_endio:
[A] Child 0 reads parent->bi_status,
    no error.
                                         Child bio 1 reads parent, no error seen
                                         It sets parent->bi_status to an error
                                         It calls bio_put.
Child bio 0 calls bio_put
[end __bio_chain_endio]                  [end __bio_chain_endio]
                                         In bio_chain_endio(), bio_endio(parent)
                                         is called, calling bio_remaining_done()
                                         which decrements __bi_remaining to 1
                                         and returns false, so no further endio
                                         stuff is done.
In bio_chain_endio(), bio_endio(parent)
is called, calling bio_remaining_done(),
decrementing parent->__bi_remaining to
 0, and continuing to finish parent.
Either for block tracing or for parent's
bi_end_io(), this thread tries to read
parent->bi_status again.

The compiler or the CPU may cache the read from [A], and since there
are no intervening barriers, parent->bi_status is still believed on
thread 0 to be success. Thus the bio may still be falsely believed to
have completed successfully, even though child 1 set an error in it.

Am I missing a subtlety here?

Either neilb's original or even Jens' suggestion would be fine though.

      if (!parent->bi_status && bio->bi_status)
              parent->bi_status = bio->bi_status;

Even if your scenario did play out (which I agree it looks possible)
it'd just degenerate to neilb's version:

      if (bio->bi_status)
              parent->bi_status = bio->bi_status;

Which also accomplishes fixing what Neil originally detailed in his
patch header.