Re: [PATCH V3] md: move bitmap_destroy before __md_stop

[Shaohua Li <shli@kernel.org>]

On Wed, Mar 08, 2017 at 10:31:32AM +0800, Guoqing Jiang wrote:
> Since we have switched to sync way to handle METADATA_UPDATED
> msg for md-cluster, then process_metadata_update is depended
> on mddev->thread->wqueue.
> 
> With the new change, clustered raid could possible hang if
> array received a METADATA_UPDATED msg after array unregistered
> mddev->thread, so we need to stop clustered raid (bitmap_destroy
> 	 -> bitmap_free -> md_cluster_stop) earlier than unregister
> thread (mddev_detach -> md_unregister_thread).
> 
> And this change should be safe for non-clustered raid since
> all writes are stopped before the destroy. Also in md_run,
> we activate the personality (pers->run()) before activating
> the bitmap (bitmap_create()). So it is pleasingly symmetric
> to stop the bitmap (bitmap_destroy()) before stopping the
> personality (__md_stop() calls pers->free()).
> 
> But we don't want to break the codes for waiting behind IO as
> Shaohua mentioned, so move those codes from mddev_detach to
> bitmap_destroy. Since we already check bitmap at the beginning
> of bitmap_destroy, just wait for behind_writes to be zero if
> it existed.
> 
> Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
> ---
> This version move waiting behind IO codes into bitmap_destroy
> so we can safely call bitmap_destroy before __md_stop now.
> 
>  drivers/md/bitmap.c |  9 +++++++++
>  drivers/md/md.c     | 13 ++-----------
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index b6fa55a3cff8..89a35bc092dd 100644
> --- a/drivers/md/bitmap.c
> +++ b/drivers/md/bitmap.c
> @@ -1771,6 +1771,15 @@ void bitmap_destroy(struct mddev *mddev)
>  	if (!bitmap) /* there was no bitmap */
>  		return;
>  
> +	/* wait for behind writes to complete */
> +	if (atomic_read(&bitmap->behind_writes) > 0) {
> +		printk(KERN_INFO "md:%s: behind writes in progress - waiting to stop.\n",
> +		       mdname(mddev));
> +		/* need to kick something here to make sure I/O goes? */
> +		wait_event(bitmap->behind_wait,
> +			   atomic_read(&bitmap->behind_writes) == 0);
> +	}
> +
>  	mutex_lock(&mddev->bitmap_info.mutex);
>  	spin_lock(&mddev->lock);
>  	mddev->bitmap = NULL; /* disconnect from the md device */
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 79a99a1c9ce7..b63ab4f33892 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -5534,15 +5534,6 @@ EXPORT_SYMBOL_GPL(md_stop_writes);
>  
>  static void mddev_detach(struct mddev *mddev)
>  {
> -	struct bitmap *bitmap = mddev->bitmap;
> -	/* wait for behind writes to complete */
> -	if (bitmap && atomic_read(&bitmap->behind_writes) > 0) {
> -		pr_debug("md:%s: behind writes in progress - waiting to stop.\n",
> -			 mdname(mddev));
> -		/* need to kick something here to make sure I/O goes? */
> -		wait_event(bitmap->behind_wait,
> -			   atomic_read(&bitmap->behind_writes) == 0);
> -	}

I think it's ok to add this part into bitmap_destroy, as we need to call
bitmap_destroy before mddev_detach. Look at the usage of mddev_detach, at in
one place (level_store()), we wait for the IO without bitmap_destroy. I think
we should keep this part code in mddev_detach. Maybe create a small function,
let both mddev_detach and bitmap_destroy call it.

>  	if (mddev->pers && mddev->pers->quiesce) {
>  		mddev->pers->quiesce(mddev, 1);
>  		mddev->pers->quiesce(mddev, 0);
> @@ -5574,8 +5565,8 @@ void md_stop(struct mddev *mddev)
>  	/* stop the array and free an attached data structures.
>  	 * This is called from dm-raid
>  	 */
> -	__md_stop(mddev);
>  	bitmap_destroy(mddev);
> +	__md_stop(mddev);

since we now always do bitmap_destroy and follow __md_stop, maybe move
bitmap_destroy to very begining of __md_stop

Thanks,
Shaohua