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.
This patch achieves two purposes:
1. Check on ->cleared and update on both ->cleared and ->word need to
be done atomically, and using spinlock could be the simplest solution.
2. Add extra check in sbitmap_deferred_clear(), to identify whether
->word has free bits.
Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")
Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
Changes from v6:
- Use guard() for locking by suggestion
- Modify comments by suggestion
Changes from v5:
- Modify commit message
- Change the fixes tag
Changes from v4:
- Add some comments according to suggestion
Changes from v3:
- Add more arguments to sbitmap_deferred_clear(), for those who
don't care about the return value, just pass 0
- Consider the situation when using sbitmap_get_shallow()
- Consider the situation when ->round_robin is true
- Modify commit message
Changes from v2:
- Modify commit message by suggestion
- Add extra check in sbitmap_deferred_clear() by suggestion
Changes from v1:
- simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
---
include/linux/sbitmap.h | 5 +++++
lib/sbitmap.c | 41 ++++++++++++++++++++++++++++++++++-------
2 files changed, 39 insertions(+), 7 deletions(-)
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..c09cdcc99471 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: serializes simultaneous updates of ->word and ->cleared
+ */
+ spinlock_t swap_lock;
} ____cacheline_aligned_in_smp;
/**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..3c00fe4b74fc 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,35 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
/*
* See if we have deferred clears that we can batch move
*/
-static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
+static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
+ unsigned int depth, unsigned int alloc_hint, bool wrap)
{
- unsigned long mask;
+ unsigned long mask, word_mask;
+ bool ret = false;
- if (!READ_ONCE(map->cleared))
- return false;
+ guard(spinlock_irqsave)(&map->swap_lock);
+
+ if (!map->cleared) {
+ if (depth > 0) {
+ word_mask = (~0UL) >> (BITS_PER_LONG - depth);
+ /*
+ * The current behavior is to always retry after moving
+ * ->cleared to word, and we change it to retry in case
+ * of any free bits. To avoid an infinite loop, we need
+ * to take wrap & alloc_hint into account, otherwise a
+ * soft lockup may occur.
+ */
+ if (!wrap && alloc_hint)
+ word_mask &= ~((1UL << alloc_hint) - 1);
+
+ if ((READ_ONCE(map->word) & word_mask) == word_mask)
+ ret = false;
+ else
+ ret = true;
+ }
+
+ return ret;
+ }
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -85,6 +108,7 @@ 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 +140,9 @@ 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);
@@ -126,7 +153,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
unsigned int i;
for (i = 0; i < sb->map_nr; i++)
- sbitmap_deferred_clear(&sb->map[i]);
+ sbitmap_deferred_clear(&sb->map[i], 0, 0, 0);
sb->depth = depth;
sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
@@ -179,7 +206,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
alloc_hint, wrap);
if (nr != -1)
break;
- if (!sbitmap_deferred_clear(map))
+ if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
break;
} while (1);
@@ -496,7 +523,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
unsigned int map_depth = __map_depth(sb, index);
unsigned long val;
- sbitmap_deferred_clear(map);
+ sbitmap_deferred_clear(map, 0, 0, 0);
val = READ_ONCE(map->word);
if (val == (1UL << (map_depth - 1)) - 1)
goto next;
--
2.34.1
