Re: [PATCH] null_blk: Fix scheduling in atomic with zoned mode

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 06, 2020 at 08:01:41PM +0900, Damien Le Moal wrote:
> Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed
> zone locking to using the potentially sleeping wait_on_bit_io()
> function. This is acceptable when memory backing is enabled as the
> device queue is in that case marked as blocking, but this triggers a
> scheduling while in atomic context with memory backing disabled.
> 
> Fix this by relying solely on the device zone spinlock for zone
> information protection without temporarily releasing this lock around
> null_process_cmd() execution in null_zone_write(). This is OK to do
> since when memory backing is disabled, command processing does not
> block and the memory backing lock nullb->lock is unused. This solution
> avoids the overhead of having to mark a zoned null_blk device queue as
> blocking when memory backing is unused.
> 
> This patch also adds comments to the zone locking code to explain the
> unusual locking scheme.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> ---
>  drivers/block/null_blk.h       |  2 +-
>  drivers/block/null_blk_zoned.c | 47 ++++++++++++++++++++++------------
>  2 files changed, 32 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/block/null_blk.h b/drivers/block/null_blk.h
> index cfd00ad40355..c24d9b5ad81a 100644
> --- a/drivers/block/null_blk.h
> +++ b/drivers/block/null_blk.h
> @@ -47,7 +47,7 @@ struct nullb_device {
>  	unsigned int nr_zones_closed;
>  	struct blk_zone *zones;
>  	sector_t zone_size_sects;
> -	spinlock_t zone_dev_lock;
> +	spinlock_t zone_lock;
>  	unsigned long *zone_locks;
>  
>  	unsigned long size; /* device size in MB */
> diff --git a/drivers/block/null_blk_zoned.c b/drivers/block/null_blk_zoned.c
> index 8775acbb4f8f..beb34b4f76b0 100644
> --- a/drivers/block/null_blk_zoned.c
> +++ b/drivers/block/null_blk_zoned.c
> @@ -46,11 +46,20 @@ int null_init_zoned_dev(struct nullb_device *dev, struct request_queue *q)
>  	if (!dev->zones)
>  		return -ENOMEM;
>  
> -	spin_lock_init(&dev->zone_dev_lock);
> -	dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL);
> -	if (!dev->zone_locks) {
> -		kvfree(dev->zones);
> -		return -ENOMEM;
> +	/*
> +	 * With memory backing, the zone_lock spinlock needs to be temporarily
> +	 * released to avoid scheduling in atomic context. To guarantee zone
> +	 * information protection, use a bitmap to lock zones with
> +	 * wait_on_bit_lock_io(). Sleeping on the lock is OK as memory backing
> +	 * implies that the queue is marked with BLK_MQ_F_BLOCKING.
> +	 */
> +	spin_lock_init(&dev->zone_lock);
> +	if (dev->memory_backed) {
> +		dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL);
> +		if (!dev->zone_locks) {
> +			kvfree(dev->zones);
> +			return -ENOMEM;
> +		}
>  	}
>  
>  	if (dev->zone_nr_conv >= dev->nr_zones) {
> @@ -137,12 +146,17 @@ void null_free_zoned_dev(struct nullb_device *dev)
>  
>  static inline void null_lock_zone(struct nullb_device *dev, unsigned int zno)
>  {
> -	wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE);
> +	if (dev->memory_backed)
> +		wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE);
> +	spin_lock_irq(&dev->zone_lock);
>  }
>  
>  static inline void null_unlock_zone(struct nullb_device *dev, unsigned int zno)
>  {
> -	clear_and_wake_up_bit(zno, dev->zone_locks);
> +	spin_unlock_irq(&dev->zone_lock);
> +
> +	if (dev->memory_backed)
> +		clear_and_wake_up_bit(zno, dev->zone_locks);
>  }
>  
>  int null_report_zones(struct gendisk *disk, sector_t sector,
> @@ -322,7 +336,6 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector,
>  		return null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors);
>  
>  	null_lock_zone(dev, zno);
> -	spin_lock(&dev->zone_dev_lock);
>  
>  	switch (zone->cond) {
>  	case BLK_ZONE_COND_FULL:
> @@ -375,9 +388,17 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector,
>  	if (zone->cond != BLK_ZONE_COND_EXP_OPEN)
>  		zone->cond = BLK_ZONE_COND_IMP_OPEN;
>  
> -	spin_unlock(&dev->zone_dev_lock);
> +	/*
> +	 * Memory backing allocation may sleep: release the zone_lock spinlock
> +	 * to avoid scheduling in atomic context. Zone operation atomicity is
> +	 * still guaranteed through the zone_locks bitmap.
> +	 */
> +	if (dev->memory_backed)
> +		spin_unlock_irq(&dev->zone_lock);
>  	ret = null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors);
> -	spin_lock(&dev->zone_dev_lock);
> +	if (dev->memory_backed)
> +		spin_lock_irq(&dev->zone_lock);

Do we actually need to take zone_lock at all for the memory_backed
case?  Should the !memory_backed just use a per-zone spinlock?