[PATCH v2] sbitmap: fix io hung due to race on sbitmap_word::cleared

Yang Yang
2024-06-04T03:12:19Z

Configuration for sbq:
depth=64, wake_batch=6, shift=6, map_nr=1

1. There are 64 requests in progress:
map->word = 0xFFFFFFFFFFFFFFFF
2. After all the 64 requests complete, and no more requests come:
map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
3. Now two tasks try to allocate requests:
T1: T2:
__blk_mq_get_tag .
__sbitmap_queue_get .
sbitmap_get .
sbitmap_find_bit .
sbitmap_find_bit_in_word .
__sbitmap_get_word -> nr=-1 __blk_mq_get_tag
sbitmap_deferred_clear __sbitmap_queue_get
/* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
return false; __sbitmap_get_word -> nr=-1
mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
atomic_long_andnot() /* map->cleared=0 */
if (!(map->cleared))
return false;
/*
* map->cleared is cleared by T1
* T2 fail to acquire the tag
*/

4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
up due to the wake_batch being set at 6. If no more requests come, T1
will wait here indefinitely.

To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
remove swap_lock"), which causes this issue.

Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <yang.yang@vivo.com>

---
Changes from v1:
- simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
---
include/linux/sbitmap.h | 5 +++++
lib/sbitmap.c | 17 ++++++++++++++---
2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..ec0b0e73c906 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -36,6 +36,11 @@ struct sbitmap_word {
* @cleared: word holding cleared bits
*/
unsigned long cleared ____cacheline_aligned_in_smp;
+
+ /**
+ * @swap_lock: Held while swapping word <-> cleared
+ */
+ spinlock_t swap_lock;
} ____cacheline_aligned_in_smp;

/**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..dee02a0266a6 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -63,9 +63,13 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
{
unsigned long mask;
+ bool ret = false;
+ unsigned long flags;

- if (!READ_ONCE(map->cleared))
- return false;
+ spin_lock_irqsave(&map->swap_lock, flags);
+
+ if (!map->cleared)
+ goto out_unlock;

/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -77,7 +81,10 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
*/
atomic_long_andnot(mask, (atomic_long_t *)&map->word);
BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
- return true;
+ ret = true;
+out_unlock:
+ spin_unlock_irqrestore(&map->swap_lock, flags);
+ return ret;
}

int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -85,6 +92,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
bool alloc_hint)
{
unsigned int bits_per_word;
+ int i;

if (shift < 0)
shift = sbitmap_calculate_shift(depth);
@@ -116,6 +124,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
return -ENOMEM;
}

+ for (i = 0; i < sb->map_nr; i++)
+ spin_lock_init(&sb->map[i].swap_lock);
+
return 0;
}
EXPORT_SYMBOL_GPL(sbitmap_init_node);
--
2.34.1