include/linux/sbitmap.h | 5 +++++ lib/sbitmap.c | 46 ++++++++++++++++++++++++++++++++++------- 2 files changed, 43 insertions(+), 8 deletions(-)
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.
So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
may cause potential race.
2. Add extra check in sbitmap_deferred_clear(), to identify whether
->word has free bits.
Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <yang.yang@vivo.com>
---
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 | 46 ++++++++++++++++++++++++++++++++++-------
2 files changed, 43 insertions(+), 8 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..22d6e86ba87f 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, flags, word_mask;
+ bool ret = false;
- if (!READ_ONCE(map->cleared))
- return false;
+ spin_lock_irqsave(&map->swap_lock, flags);
+
+ 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 dead loop, we need to take
+ * wrap & alloc_hint into account. Without this, a soft
+ * lockup was detected in our test environment.
+ */
+ if (!wrap && alloc_hint)
+ word_mask &= ~((1UL << alloc_hint) - 1);
+
+ if ((READ_ONCE(map->word) & word_mask) == word_mask)
+ ret = false;
+ else
+ ret = true;
+ }
+
+ goto out_unlock;
+ }
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -77,7 +100,10 @@ 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 +111,7 @@ 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 +143,9 @@ 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 +156,7 @@ 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 +209,7 @@ 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 +526,7 @@ 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;
--
2.34.1On 7/3/24 03:28, 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:
> 1. 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.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Is it blamed right? Considering that the revert alone doesn't fix
the problem, it sounds like the 2nd step might need to be ported
to kernels even without the blamed commit.
--
Pavel Begunkov
On 2024/7/3 19:55, Pavel Begunkov wrote:
> On 7/3/24 03:28, 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:
>> 1. 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.
>>
>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>> ->word has free bits.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>
> Is it blamed right? Considering that the revert alone doesn't fix
> the problem, it sounds like the 2nd step might need to be ported
> to kernels even without the blamed commit.
>
Got it. How would you feel about removing
"Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")" and modifying
the commit message as follows?
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.
Thanks.
On 7/3/24 13:28, YangYang wrote:
> On 2024/7/3 19:55, Pavel Begunkov wrote:
>> On 7/3/24 03:28, 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:
>>> 1. 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.
>>>
>>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>>> ->word has free bits.
>>>
>>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>>
>> Is it blamed right? Considering that the revert alone doesn't fix
>> the problem, it sounds like the 2nd step might need to be ported
>> to kernels even without the blamed commit.
>>
>
> Got it. How would you feel about removing
> "Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")" and modifying
> the commit message as follows?
We need a fixes tag, which commit caused it? Can it happen prior
to ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")?
> 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.
>
> Thanks.
--
Pavel Begunkov
On 2024/7/5 18:58, Pavel Begunkov wrote:
> On 7/3/24 13:28, YangYang wrote:
>> On 2024/7/3 19:55, Pavel Begunkov wrote:
>>> On 7/3/24 03:28, 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:
>>>> 1. 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.
>>>>
>>>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>>>> ->word has free bits.
>>>>
>>>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>>>
>>> Is it blamed right? Considering that the revert alone doesn't fix
>>> the problem, it sounds like the 2nd step might need to be ported
>>> to kernels even without the blamed commit.
>>>
>>
>> Got it. How would you feel about removing
>> "Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")" and modifying
>> the commit message as follows?
>
> We need a fixes tag, which commit caused it? Can it happen prior
> to ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")?
The initial introduction of the sbitmap_deferred_clear function also
introduced a potential race condition.
So the fixes tag should be:
Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")
Thanks.
>
>> 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.
>>
>> Thanks.
>
On Wed, Jul 3, 2024 at 10:28 AM Yang Yang <yang.yang@vivo.com> 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:
> 1. 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.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <yang.yang@vivo.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
© 2016 - 2025 Red Hat, Inc.