From: John Dorminy <jdorminy@redhat.com>
To: Mike Snitzer <snitzer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, NeilBrown <neilb@suse.com>,
linux-block@vger.kernel.org,
device-mapper development <dm-devel@redhat.com>,
Milan Broz <gmazyland@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: block: be more careful about status in __bio_chain_endio
Date: Fri, 22 Feb 2019 22:10:39 -0500 [thread overview]
Message-ID: <CAMeeMh9qLkTByWJewPR4o844wPkA-g5Hnm7aGjszuPopPAe8vA@mail.gmail.com> (raw)
In-Reply-To: <20190223024402.GA12407@redhat.com>
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.]
next prev parent reply other threads:[~2019-02-23 3:10 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <70cda2a3-f246-d45b-f600-1f9d15ba22ff@gmail.com>
[not found] ` <87eflmpqkb.fsf@notabene.neil.brown.name>
2019-02-22 21:10 ` block: be more careful about status in __bio_chain_endio Mike Snitzer
2019-02-22 22:46 ` Jens Axboe
2019-02-22 23:55 ` Mike Snitzer
2019-02-23 2:02 ` John Dorminy
2019-02-23 2:44 ` Mike Snitzer
2019-02-23 3:10 ` John Dorminy [this message]
2019-06-12 2:56 ` John Dorminy
2019-06-12 7:01 ` Christoph Hellwig
2019-06-17 7:32 ` Hannes Reinecke
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