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SubjectRe: [PATCH v2] block: fix mis-synchronisation in blkdev_issue_zeroout()
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Lukas Czerner <lczerner@redhat.com> writes:

> BZ29402
> https://bugzilla.kernel.org/show_bug.cgi?id=29402
>
> We can hit serious mis-synchronization in bio completion path of
> blkdev_issue_zeroout() leading to a panic.
>
> The problem is that when we are going to wait_for_completion() in
> blkdev_issue_zeroout() we check if the bb.done equals issued (number of
> submitted bios). If it does, we can skip the wait_for_completition()
> and just out of the function since there is nothing to wait for.
> However, there is a ordering problem because bio_batch_end_io() is
> calling atomic_inc(&bb->done) before complete(), hence it might seem to
> blkdev_issue_zeroout() that all bios has been completed and exit. At
> this point when bio_batch_end_io() is going to call complete(bb->wait),
> bb and wait does not longer exist since it was allocated on stack in
> blkdev_issue_zeroout() ==> panic!
>
> (thread 1) (thread 2)
> bio_batch_end_io() blkdev_issue_zeroout()
> if(bb) { ...
> if (bb->end_io) ...
> bb->end_io(bio, err); ...
> atomic_inc(&bb->done); ...
> ... while (issued != atomic_read(&bb.done))
> ... (let issued == bb.done)
> ... (do the rest of the function)
> ... return ret;
> complete(bb->wait);
> ^^^^^^^^
> panic
>
> We can fix this easily by simplifying bio_batch and completion counting.
>
> Also remove bio_end_io_t *end_io since it is not used.
>
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> Reported-by: Eric Whitney <eric.whitney@hp.com>
> Tested-by: Eric Whitney <eric.whitney@hp.com>
> CC: Jens Axboe <axboe@kernel.dk>
> CC: Dmitry Monakhov <dmonakhov@openvz.org>
> CC: Jeff Moyer <jmoyer@redhat.com>
> ---
> block/blk-lib.c | 19 +++++++------------
> 1 files changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index eec78be..bd3e8df 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -109,7 +109,6 @@ struct bio_batch
> atomic_t done;
> unsigned long flags;
> struct completion *wait;
> - bio_end_io_t *end_io;
> };
>
> static void bio_batch_end_io(struct bio *bio, int err)
> @@ -122,12 +121,9 @@ static void bio_batch_end_io(struct bio *bio, int err)
> else
> clear_bit(BIO_UPTODATE, &bb->flags);
> }
> - if (bb) {
> - if (bb->end_io)
> - bb->end_io(bio, err);
> - atomic_inc(&bb->done);
> - complete(bb->wait);
> - }
> + if (bb)
> + if (atomic_dec_and_test(&bb->done))
> + complete(bb->wait);

I think bb will always be set here, no real need to check.

Anyway, I though I already added my:

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>

to this. No?

Cheers,
Jeff


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