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:
First:
Check on ->cleared and update on both ->cleared and ->word need to be
done atomically, and using spinlock could be the simplest solution.
So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
may cause potential race.
Second:
Add extra check in sbitmap_deferred_clear(), to identify whether
map->cleared is cleared by another task after failing to get a tag.
Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <[email protected]>
---
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 | 28 +++++++++++++++++++++-------
2 files changed, 26 insertions(+), 7 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..06b837311e03 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,19 @@ 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 long mask;
+ unsigned long flags;
+ bool ret;
- if (!READ_ONCE(map->cleared))
- return false;
+ spin_lock_irqsave(&map->swap_lock, flags);
+
+ if (!map->cleared) {
+ ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
+ goto out_unlock;
+ }
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -77,7 +84,10 @@ 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 +95,7 @@ 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 +127,9 @@ 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 +140,7 @@ 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], depth);
sb->depth = depth;
sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
@@ -179,7 +193,7 @@ 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))
break;
} while (1);
@@ -496,7 +510,7 @@ 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, map_depth);
val = READ_ONCE(map->word);
if (val == (1UL << (map_depth - 1)) - 1)
goto next;
--
2.34.1
On Thu, Jun 06, 2024 at 09:57:21PM +0800, Yang Yang wrote:
> 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:
> First:
> Check on ->cleared and update on both ->cleared and ->word need to be
> done atomically, and using spinlock could be the simplest solution.
> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> may cause potential race.
>
> Second:
> Add extra check in sbitmap_deferred_clear(), to identify whether
> map->cleared is cleared by another task after failing to get a tag.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <[email protected]>
>
> ---
> 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 | 28 +++++++++++++++++++++-------
> 2 files changed, 26 insertions(+), 7 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..06b837311e03 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,19 @@ 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 long mask;
> + unsigned long flags;
> + bool ret;
>
> - if (!READ_ONCE(map->cleared))
> - return false;
> + spin_lock_irqsave(&map->swap_lock, flags);
> +
> + if (!map->cleared) {
> + ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
> + goto out_unlock;
> + }
Direct check over map->word should be more efficient than find_first_zero_bit():
if (READ_ONCE(map->word) == (1UL << depth) - 1)
ret = false;
else
ret = true;
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,7 +84,10 @@ 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 +95,7 @@ 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 +127,9 @@ 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 +140,7 @@ 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], depth);
The above 'depth' is whole sbitmap depth, and you need to figure out
word depth from __map_depth().
Thanks,
Ming
On 2024/6/7 17:44, Ming Lei wrote:
> On Thu, Jun 06, 2024 at 09:57:21PM +0800, Yang Yang wrote:
>> 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:
>> First:
>> Check on ->cleared and update on both ->cleared and ->word need to be
>> done atomically, and using spinlock could be the simplest solution.
>> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
>> may cause potential race.
>>
>> Second:
>> Add extra check in sbitmap_deferred_clear(), to identify whether
>> map->cleared is cleared by another task after failing to get a tag.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>> Signed-off-by: Yang Yang <[email protected]>
>>
>> ---
>> 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 | 28 +++++++++++++++++++++-------
>> 2 files changed, 26 insertions(+), 7 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..06b837311e03 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -60,12 +60,19 @@ 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 long mask;
>> + unsigned long flags;
>> + bool ret;
>>
>> - if (!READ_ONCE(map->cleared))
>> - return false;
>> + spin_lock_irqsave(&map->swap_lock, flags);
>> +
>> + if (!map->cleared) {
>> + ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
>> + goto out_unlock;
>> + }
>
> Direct check over map->word should be more efficient than find_first_zero_bit():
>
> if (READ_ONCE(map->word) == (1UL << depth) - 1)
> ret = false;
> else
> ret = true;
Got it. A little modified as follows, because:
1. The depth may be shallow_depth, which is less than __map_depth()
2. (1UL << depth) may overflow
72 #if BITS_PER_LONG == 64
73 unsigned long word_mask = U64_MAX >> (BITS_PER_LONG - depth);
74 #else
75 unsigned long word_mask = U32_MAX >> (BITS_PER_LONG - depth);
76 #endif
77
78 if ((READ_ONCE(map->word) & word_mask) == word_mask)
79 ret = false;
80 else
81 ret = true;
>>
>> /*
>> * First get a stable cleared mask, setting the old mask to 0.
>> @@ -77,7 +84,10 @@ 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 +95,7 @@ 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 +127,9 @@ 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 +140,7 @@ 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], depth);
>
> The above 'depth' is whole sbitmap depth, and you need to figure out
> word depth from __map_depth().
Got it.
Thanks.