linux-kernel - Re: block: be more careful about status in __bio_chain

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Message-ID: <CAMeeMh9qLkTByWJewPR4o844wPkA-g5Hnm7aGjszuPopPAe8vA@mail.gmail.com>
Date:   Fri, 22 Feb 2019 22:10:39 -0500
From:   John Dorminy <jdorminy@...hat.com>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.com>,
        linux-block@...r.kernel.org,
        device-mapper development <dm-devel@...hat.com>,
        Milan Broz <gmazyland@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: block: be more careful about status in __bio_chain_endio

I'm also worried about the other two versions, though:

memory-barriers.txt#1724:

1724 (*) The compiler is within its rights to invent stores to a variable,

i.e. the compiler is free to decide __bio_chain_endio looks like this:

static struct bio *__bio_chain_endio(struct bio *bio)
{
  struct bio *parent = bio->bi_private;
  blk_status_t tmp = parent->bi_status;
  parent->bi_status = bio->bi_status;
  if (!bio->bi_status)
    parent->bi_status = tmp;
  bio_put(bio);
  return parent;
}

In which case, the read and later store on the two different threads
may overlap in such a way that bio_endio sometimes sees success, even
if one child had an error.

As a result, I believe the setting of parent->bi_status needs to be a
WRITE_ONCE() and the later reading needs to be a READ_ONCE()
[although, since the later reading happens in many different
functions, perhaps some other barrier to make sure all readers get the
correct value is in order.]