From: Mike Snitzer <snitzer@redhat.com>
To: John Dorminy <jdorminy@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 21:44:02 -0500 [thread overview]
Message-ID: <20190223024402.GA12407@redhat.com> (raw)
In-Reply-To: <CAMeeMh-2ANOr_Sb66EyA_HULkVRudD7fyOZsDbpRpDrshwnR2w@mail.gmail.com>
On Fri, Feb 22 2019 at 9:02pm -0500,
John Dorminy <jdorminy@redhat.com> 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.
next prev parent reply other threads:[~2019-02-23 2:44 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 [this message]
2019-02-23 3:10 ` John Dorminy
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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