Re: [PATCH v2 5/5] btrfs: raid56: Use bio_counter to protect scrub

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

On Fri, Mar 24, 2017 at 10:00:27AM +0800, Qu Wenruo wrote:
> Unlike other place calling btrfs_map_block(), in raid56 scrub, we don't
> use bio_counter to protect from race against dev replace.
> 
> This patch will use bio_counter to protect from the beginning of calling
> btrfs_map_sblock(), until rbio endio.
> 
> Liu Bo <bo.li.liu@oracle.com>
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> ---
>  fs/btrfs/raid56.c | 2 ++
>  fs/btrfs/scrub.c  | 5 +++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> index 1571bf26dc07..3a083165400f 100644
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -2642,6 +2642,7 @@ static void async_scrub_parity(struct btrfs_raid_bio *rbio)
>  
>  void raid56_parity_submit_scrub_rbio(struct btrfs_raid_bio *rbio)
>  {
> +	rbio->generic_bio_cnt = 1;

To keep consistent with other places, can you please do this setting when
allocating rbio?

>  	if (!lock_stripe_add(rbio))
>  		async_scrub_parity(rbio);
>  }
> @@ -2694,6 +2695,7 @@ static void async_missing_raid56(struct btrfs_raid_bio *rbio)
>  
>  void raid56_submit_missing_rbio(struct btrfs_raid_bio *rbio)
>  {
> +	rbio->generic_bio_cnt = 1;
>  	if (!lock_stripe_add(rbio))
>  		async_missing_raid56(rbio);
>  }

Ditto.

> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index 2a5458004279..265387bf3af8 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -2379,6 +2379,7 @@ static void scrub_missing_raid56_pages(struct scrub_block *sblock)
>  	int ret;
>  	int i;
>  
> +	btrfs_bio_counter_inc_blocked(fs_info);
>  	ret = btrfs_map_sblock(fs_info, BTRFS_MAP_GET_READ_MIRRORS, logical,
>  			&length, &bbio, 0, 1);
>  	if (ret || !bbio || !bbio->raid_map)
> @@ -2423,6 +2424,7 @@ static void scrub_missing_raid56_pages(struct scrub_block *sblock)
>  rbio_out:
>  	bio_put(bio);
>  bbio_out:
> +	btrfs_bio_counter_dec(fs_info);
>  	btrfs_put_bbio(bbio);
>  	spin_lock(&sctx->stat_lock);
>  	sctx->stat.malloc_errors++;
> @@ -2966,6 +2968,8 @@ static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
>  		goto out;
>  
>  	length = sparity->logic_end - sparity->logic_start;
> +
> +	btrfs_bio_counter_inc_blocked(fs_info);
>  	ret = btrfs_map_sblock(fs_info, BTRFS_MAP_WRITE, sparity->logic_start,
>  			       &length, &bbio, 0, 1);
>  	if (ret || !bbio || !bbio->raid_map)
> @@ -2993,6 +2997,7 @@ static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
>  rbio_out:
>  	bio_put(bio);
>  bbio_out:
> +	btrfs_bio_counter_dec(fs_info);
>  	btrfs_put_bbio(bbio);
>  	bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
>  		  sparity->nsectors);
> -- 
> 2.12.1
> 
> 
>

If patch 4 and 5 are still supposed to fix the same problem, can you please
merge them into one patch so that a future bisect could be precise?

And while I believe this fixes the crash described in patch 4,
scrub_setup_recheck_block() also retrives all stripes, and if we scrub
one device, and another device is being replaced so it could be freed
during scrub, is it another potential race case?

Thanks,

-liubo