[PATCH 5.9 251/255] null_blk: Fix scheduling in atomic with zoned mode

Greg Kroah-Hartman
2020-11-17T13:43:05Z

From: Damien Le Moal <damien.lemoal@wdc.com>

commit e1777d099728a76a8f8090f89649aac961e7e530 upstream.

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.

Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/block/null_blk.h | 1 +
drivers/block/null_blk_zoned.c | 31 +++++++++++++++++++++++++------
2 files changed, 26 insertions(+), 6 deletions(-)

--- a/drivers/block/null_blk.h
+++ b/drivers/block/null_blk.h
@@ -44,6 +44,7 @@ struct nullb_device {
unsigned int nr_zones;
struct blk_zone *zones;
sector_t zone_size_sects;
+ spinlock_t zone_lock;
unsigned long *zone_locks;

unsigned long size; /* device size in MB */
--- a/drivers/block/null_blk_zoned.c
+++ b/drivers/block/null_blk_zoned.c
@@ -46,10 +46,20 @@ int null_init_zoned_dev(struct nullb_dev
if (!dev->zones)
return -ENOMEM;

- 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) {
@@ -118,12 +128,16 @@ void null_free_zoned_dev(struct nullb_de

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,
@@ -233,7 +247,12 @@ static blk_status_t null_zone_write(stru
if (zone->cond != BLK_ZONE_COND_EXP_OPEN)
zone->cond = BLK_ZONE_COND_IMP_OPEN;

+ if (dev->memory_backed)
+ spin_unlock_irq(&dev->zone_lock);
ret = null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors);
+ if (dev->memory_backed)
+ spin_lock_irq(&dev->zone_lock);
+
if (ret != BLK_STS_OK)
break;