linux-kernel - Re: false positive lockdep splat with loop device

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Date:   Tue, 10 Oct 2017 11:16:03 +0200
From:   Ilya Dryomov <idryomov@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        xfs <linux-xfs@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Byungchul Park <byungchul.park@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>
Subject: Re: false positive lockdep splat with loop device

On Thu, Oct 5, 2017 at 6:33 PM, Christoph Hellwig <hch@...radead.org> wrote:
> Does the patch below fix the warning for you?
>
> --
> From 28aae7104425433d39e6142adcd5b88dc5b0ad5f Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@....de>
> Date: Thu, 5 Oct 2017 18:31:02 +0200
> Subject: block: use DECLARE_COMPLETION_ONSTACK in submit_bio_wait
>
> This way we get our lockdep annotations right in case multiple layers
> in the stack use submit_bio_wait.
>
> It also happens to simplify the code by getting rid of the submit_bio_ret
> structure.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  block/bio.c | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 8338304ea256..4e18e959fc0a 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -917,17 +917,9 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  }
>  EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
>
> -struct submit_bio_ret {
> -       struct completion event;
> -       int error;
> -};
> -
>  static void submit_bio_wait_endio(struct bio *bio)
>  {
> -       struct submit_bio_ret *ret = bio->bi_private;
> -
> -       ret->error = blk_status_to_errno(bio->bi_status);
> -       complete(&ret->event);
> +       complete(bio->bi_private);
>  }
>
>  /**
> @@ -943,16 +935,15 @@ static void submit_bio_wait_endio(struct bio *bio)
>   */
>  int submit_bio_wait(struct bio *bio)
>  {
> -       struct submit_bio_ret ret;
> +       DECLARE_COMPLETION_ONSTACK(done);
>
> -       init_completion(&ret.event);
> -       bio->bi_private = &ret;
> +       bio->bi_private = &done;
>         bio->bi_end_io = submit_bio_wait_endio;
>         bio->bi_opf |= REQ_SYNC;
>         submit_bio(bio);
> -       wait_for_completion_io(&ret.event);
> +       wait_for_completion_io(&done);
>
> -       return ret.error;
> +       return blk_status_to_errno(bio->bi_status);
>  }
>  EXPORT_SYMBOL(submit_bio_wait);

No, it doesn't -- the splat is a little bit more complicated, but
fundamentally the same thing.

Thanks,

                Ilya

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