Re: [md PATCH 02/15] md/raid5: simplfy delaying of writes while metadata is updated.

[Shaohua Li <shli@kernel.org>]

On Wed, Mar 15, 2017 at 02:05:12PM +1100, Neil Brown wrote:
> If a device fails during a write, we must ensure the failure is
> recorded in the metadata before the completion of the write is
> acknowleged.
> 
> Commit c3cce6cda162 ("md/raid5: ensure device failure recorded before
> write request returns.")  added code for this, but it was
> unnecessarily complicated.  We already had similar functionality for
> handling updates to the bad-block-list, thanks to Commit de393cdea66c
> ("md: make it easier to wait for bad blocks to be acknowledged.")
> 
> So revert most of the former commit, and instead avoid collecting
> completed writes if MD_CHANGE_PENDING is set.  raid5d() will then flush
> the metadata and retry the stripe_head.
> As this change can leave a stripe_head ready for handling immediately
> after handle_active_stripes() returns, we change raid5_do_work() to
> pause when MD_CHANGE_PENDING is set, so that it doesn't spin.
> 
> We check MD_CHANGE_PENDING *after* analyse_stripe() as it could be set
> asynchronously.  After analyse_stripe(), we have collected stable data
> about the state of devices, which will be used to make decisions.
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  drivers/md/raid5.c |   31 ++++++++-----------------------
>  drivers/md/raid5.h |    3 ---
>  2 files changed, 8 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index cc2d039b4aae..f990f74901d2 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -4690,7 +4690,8 @@ static void handle_stripe(struct stripe_head *sh)
>  	if (test_bit(STRIPE_LOG_TRAPPED, &sh->state))
>  		goto finish;
>  
> -	if (s.handle_bad_blocks) {
> +	if (s.handle_bad_blocks ||
> +	    test_bit(MD_SB_CHANGE_PENDING, &conf->mddev->sb_flags)) {
>  		set_bit(STRIPE_HANDLE, &sh->state);
>  		goto finish;
>  	}
> @@ -5020,15 +5021,8 @@ static void handle_stripe(struct stripe_head *sh)
>  			md_wakeup_thread(conf->mddev->thread);
>  	}
>  
> -	if (!bio_list_empty(&s.return_bi)) {
> -		if (test_bit(MD_SB_CHANGE_PENDING, &conf->mddev->sb_flags)) {
> -			spin_lock_irq(&conf->device_lock);
> -			bio_list_merge(&conf->return_bi, &s.return_bi);
> -			spin_unlock_irq(&conf->device_lock);
> -			md_wakeup_thread(conf->mddev->thread);
> -		} else
> -			return_io(&s.return_bi);
> -	}
> +	if (!bio_list_empty(&s.return_bi))
> +		return_io(&s.return_bi);
>  
>  	clear_bit_unlock(STRIPE_ACTIVE, &sh->state);
>  }
> @@ -6225,6 +6219,7 @@ static void raid5_do_work(struct work_struct *work)
>  	struct r5worker *worker = container_of(work, struct r5worker, work);
>  	struct r5worker_group *group = worker->group;
>  	struct r5conf *conf = group->conf;
> +	struct mddev *mddev = conf->mddev;
>  	int group_id = group - conf->worker_groups;
>  	int handled;
>  	struct blk_plug plug;
> @@ -6245,6 +6240,9 @@ static void raid5_do_work(struct work_struct *work)
>  		if (!batch_size && !released)
>  			break;
>  		handled += batch_size;
> +		wait_event_lock_irq(mddev->sb_wait,
> +				    !test_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags),
MD_SB_CHANGE_PENDING?

> +				    conf->device_lock);
>  	}
>  	pr_debug("%d stripes handled\n", handled);
>  
> @@ -6272,18 +6270,6 @@ static void raid5d(struct md_thread *thread)
>  
>  	md_check_recovery(mddev);
>  
> -	if (!bio_list_empty(&conf->return_bi) &&
> -	    !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
> -		struct bio_list tmp = BIO_EMPTY_LIST;
> -		spin_lock_irq(&conf->device_lock);
> -		if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
> -			bio_list_merge(&tmp, &conf->return_bi);
> -			bio_list_init(&conf->return_bi);
> -		}
> -		spin_unlock_irq(&conf->device_lock);
> -		return_io(&tmp);
> -	}
> -
>  	blk_start_plug(&plug);
>  	handled = 0;
>  	spin_lock_irq(&conf->device_lock);
> @@ -6935,7 +6921,6 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>  	INIT_LIST_HEAD(&conf->hold_list);
>  	INIT_LIST_HEAD(&conf->delayed_list);
>  	INIT_LIST_HEAD(&conf->bitmap_list);
> -	bio_list_init(&conf->return_bi);
>  	init_llist_head(&conf->released_stripes);
>  	atomic_set(&conf->active_stripes, 0);
>  	atomic_set(&conf->preread_active_stripes, 0);
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index ba5b7a3790af..13800dc9dd88 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -638,9 +638,6 @@ struct r5conf {
>  	int			skip_copy; /* Don't copy data from bio to stripe cache */
>  	struct list_head	*last_hold; /* detect hold_list promotions */
>  
> -	/* bios to have bi_end_io called after metadata is synced */
> -	struct bio_list		return_bi;
> -
>  	atomic_t		reshape_stripes; /* stripes with pending writes for reshape */
>  	/* unfortunately we need two cache names as we temporarily have
>  	 * two caches.
> 
>