Re: [PATCH 6/7] Btrfs: fix deadlock when mounting a degraded fs

[Liu Bo <bo.li.liu@oracle.com>]

On Thu, Jun 19, 2014 at 10:42:54AM +0800, Miao Xie wrote:
> The deadlock happened when we mount degraded filesystem, the reproduced
> steps are following:
>  # mkfs.btrfs -f -m raid1 -d raid1 <dev0> <dev1>
>  # echo 1 > /sys/block/`basename <dev0>`/device/delete
>  # mount -o degraded <dev1> <mnt>
> 
> The reason was that the counter -- bi_remaining was wrong. If the missing
> or unwriteable device was the last device in the mapping array, we would
> not submit the original bio, so we shouldn't increase bi_remaining of it
> in btrfs_end_bio(), or we would skip the final endio handle.
> 
> Fix this problem by adding a flag into btrfs bio structure. If we submit
> the original bio, we will set the flag, and we increase bi_remaining counter,
> or we don't.
> 
> Though there is another way to fix it -- decrease bi_remaining counter of the
> original bio when we make sure the original bio is not submitted, this method
> need add more check and is easy to make mistake.

Happen to look at this problem, looks good to me.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

-liubo

> 
> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
> ---
>  fs/btrfs/volumes.c | 7 ++++++-
>  fs/btrfs/volumes.h | 3 +++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 31f9036..4ca3c92 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5415,8 +5415,12 @@ static void btrfs_end_bio(struct bio *bio, int err)
>  			set_bit(BIO_UPTODATE, &bio->bi_flags);
>  			err = 0;
>  		}
> +
> +		if (likely(bbio->flags & BTRFS_BIO_ORIG_BIO_SUBMITTED))
> +			bio_endio_nodec(bio, err);
> +		else
> +			bio_endio(bio, err);
>  		kfree(bbio);
> -		bio_endio_nodec(bio, err);
>  	} else if (!is_orig_bio) {
>  		bio_put(bio);
>  	}
> @@ -5671,6 +5675,7 @@ int btrfs_map_bio(struct btrfs_root *root, int rw, struct bio *bio,
>  			BUG_ON(!bio); /* -ENOMEM */
>  		} else {
>  			bio = first_bio;
> +			bbio->flags |= BTRFS_BIO_ORIG_BIO_SUBMITTED;
>  		}
>  
>  		submit_stripe_bio(root, bbio, bio,
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 1a15bbe..2aaa00c 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -190,11 +190,14 @@ struct btrfs_bio_stripe {
>  struct btrfs_bio;
>  typedef void (btrfs_bio_end_io_t) (struct btrfs_bio *bio, int err);
>  
> +#define BTRFS_BIO_ORIG_BIO_SUBMITTED	0x1
> +
>  struct btrfs_bio {
>  	atomic_t stripes_pending;
>  	struct btrfs_fs_info *fs_info;
>  	bio_end_io_t *end_io;
>  	struct bio *orig_bio;
> +	unsigned long flags;
>  	void *private;
>  	atomic_t error;
>  	int max_errors;
> -- 
> 1.9.3
> 
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