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SubjectRe: false positive lockdep splat with loop device
Does the patch below fix the warning for you?

--
From 28aae7104425433d39e6142adcd5b88dc5b0ad5f Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.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@lst.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);

--
2.14.1
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