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.
Fix this issue by adding a new flag swap_inprogress to indicate whether
the swap is ongoing.
Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <[email protected]>
---
include/linux/sbitmap.h | 5 +++++
lib/sbitmap.c | 22 ++++++++++++++++++++--
2 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
+ */
+ atomic_t swap_inprogress;
} ____cacheline_aligned_in_smp;
/**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..d4bb258fe8b0 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
*/
static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
{
- unsigned long mask;
+ unsigned long mask, flags;
+ int zero = 0;
- if (!READ_ONCE(map->cleared))
+ if (!READ_ONCE(map->cleared)) {
+ if (atomic_read(&map->swap_inprogress))
+ goto out_wait;
return false;
+ }
+
+ if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
+ goto out_wait;
+
+ local_irq_save(flags);
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -77,6 +86,15 @@ 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));
+
+ atomic_set(&map->swap_inprogress, 0);
+ smp_mb__after_atomic();
+ local_irq_restore(flags);
+ return true;
+
+out_wait:
+ while (atomic_read(&map->swap_inprogress))
+ ;
return true;
}
--
2.34.1
On 2024/5/27 12:26, 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.
Right at this moment, if a new task attempts to switch the elevator, these
tasks may hang forever.
T2: T3: T4:
blk_mq_submit_bio . .
__blk_mq_alloc_requests . .
blk_mq_get_tag . .
io_schedule . .
elevator_switch .
blk_mq_freeze_queue .
blk_freeze_queue_start .
q->mq_freeze_depth=1 .
blk_mq_freeze_queue_wait .
blk_mq_submit_bio
__bio_queue_enter
wait_event(!q->mq_freeze_depth)
>
> Fix this issue by adding a new flag swap_inprogress to indicate whether
> the swap is ongoing.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <[email protected]>
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 22 ++++++++++++++++++++--
> 2 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
> + */
> + atomic_t swap_inprogress;
> } ____cacheline_aligned_in_smp;
>
> /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d4bb258fe8b0 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> */
> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> {
> - unsigned long mask;
> + unsigned long mask, flags;
> + int zero = 0;
>
> - if (!READ_ONCE(map->cleared))
> + if (!READ_ONCE(map->cleared)) {
> + if (atomic_read(&map->swap_inprogress))
> + goto out_wait;
> return false;
> + }
> +
> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
> + goto out_wait;
> +
> + local_irq_save(flags);
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,6 +86,15 @@ 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));
> +
> + atomic_set(&map->swap_inprogress, 0);
> + smp_mb__after_atomic();
> + local_irq_restore(flags);
> + return true;
> +
> +out_wait:
> + while (atomic_read(&map->swap_inprogress))
> + ;
> return true;
> }
>
Hi,
?? 2024/05/27 12:26, Yang Yang д??:
> 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
> */
This looks correct.
>
> 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.
>
> Fix this issue by adding a new flag swap_inprogress to indicate whether
> the swap is ongoing.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <[email protected]>
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 22 ++++++++++++++++++++--
> 2 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
> + */
> + atomic_t swap_inprogress;
> } ____cacheline_aligned_in_smp;
>
> /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d4bb258fe8b0 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> */
> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> {
> - unsigned long mask;
> + unsigned long mask, flags;
> + int zero = 0;
>
> - if (!READ_ONCE(map->cleared))
> + if (!READ_ONCE(map->cleared)) {
> + if (atomic_read(&map->swap_inprogress))
> + goto out_wait;
> return false;
> + }
> +
> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
> + goto out_wait;
> +
> + local_irq_save(flags);
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,6 +86,15 @@ 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));
> +
> + atomic_set(&map->swap_inprogress, 0);
> + smp_mb__after_atomic();
> + local_irq_restore(flags);
> + return true;
> +
> +out_wait:
> + while (atomic_read(&map->swap_inprogress))
> + ;
However, I really don't like this change.
How about following patch? I think it can fix the prblem as well.
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..d1b9bd61d296 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -175,12 +175,14 @@ static int sbitmap_find_bit_in_word(struct
sbitmap_word *map,
int nr;
do {
+ bool clear = READ_ONCE(map->cleared) != 0;
+
nr = __sbitmap_get_word(&map->word, depth,
alloc_hint, wrap);
- if (nr != -1)
- break;
- if (!sbitmap_deferred_clear(map))
+ if (nr != -1 || !clear)
break;
+
+ sbitmap_deferred_clear(map);
} while (1);
return nr;
Thanks,
Kuai
> return true;
> }
>
>
On 2024/5/30 9:49, Yu Kuai wrote:
> Hi,
>
> 在 2024/05/27 12:26, Yang Yang 写道:
>> 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
>> */
>
> This looks correct.
>
>>
>> 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.
>>
>> Fix this issue by adding a new flag swap_inprogress to indicate whether
>> the swap is ongoing.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>> Signed-off-by: Yang Yang <[email protected]>
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 22 ++++++++++++++++++++--
>> 2 files changed, 25 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
>> + */
>> + atomic_t swap_inprogress;
>> } ____cacheline_aligned_in_smp;
>> /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..d4bb258fe8b0 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>> */
>> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> {
>> - unsigned long mask;
>> + unsigned long mask, flags;
>> + int zero = 0;
>> - if (!READ_ONCE(map->cleared))
>> + if (!READ_ONCE(map->cleared)) {
>> + if (atomic_read(&map->swap_inprogress))
>> + goto out_wait;
>> return false;
>> + }
>> +
>> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
>> + goto out_wait;
>> +
>> + local_irq_save(flags);
>> /*
>> * First get a stable cleared mask, setting the old mask to 0.
>> @@ -77,6 +86,15 @@ 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));
>> +
>> + atomic_set(&map->swap_inprogress, 0);
>> + smp_mb__after_atomic();
>> + local_irq_restore(flags);
>> + return true;
>> +
>> +out_wait:
>> + while (atomic_read(&map->swap_inprogress))
>> + ;
>
> However, I really don't like this change.
>
> How about following patch? I think it can fix the prblem as well.
The following patch cannot fix the problem.
Thanks.
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d1b9bd61d296 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -175,12 +175,14 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
> int nr;
>
> do {
> + bool clear = READ_ONCE(map->cleared) != 0;
> +
> nr = __sbitmap_get_word(&map->word, depth,
> alloc_hint, wrap);
> - if (nr != -1)
> - break;
> - if (!sbitmap_deferred_clear(map))
> + if (nr != -1 || !clear)
> break;
> +
> + sbitmap_deferred_clear(map);
> } while (1);
>
> return nr;
>
> Thanks,
> Kuai
>> return true;
>> }
>>
>
On Mon, May 27, 2024 at 12:26:50PM +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.
>
> Fix this issue by adding a new flag swap_inprogress to indicate whether
> the swap is ongoing.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Indeed, commit 661d4f55a794 ("sbitmap: remove swap_lock") causes this
issue, and ->cleared and ->word should have been checked & updated
atomically.
> Signed-off-by: Yang Yang <[email protected]>
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 22 ++++++++++++++++++++--
> 2 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
> + */
> + atomic_t swap_inprogress;
> } ____cacheline_aligned_in_smp;
>
> /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d4bb258fe8b0 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> */
> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> {
> - unsigned long mask;
> + unsigned long mask, flags;
> + int zero = 0;
>
> - if (!READ_ONCE(map->cleared))
> + if (!READ_ONCE(map->cleared)) {
> + if (atomic_read(&map->swap_inprogress))
> + goto out_wait;
> return false;
> + }
> +
> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
> + goto out_wait;
> +
> + local_irq_save(flags);
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,6 +86,15 @@ 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));
> +
> + atomic_set(&map->swap_inprogress, 0);
> + smp_mb__after_atomic();
> + local_irq_restore(flags);
> + return true;
> +
> +out_wait:
> + while (atomic_read(&map->swap_inprogress))
> + ;
> return true;
> }
IMO, the above fix looks a bit complicated, and I'd suggest to revert
661d4f55a794 ("sbitmap: remove swap_lock") if no clean & simple solution
can be figured out.
Thanks,
Ming
On 2024/6/4 9:23, Ming Lei wrote:
> On Mon, May 27, 2024 at 12:26:50PM +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.
>>
>> Fix this issue by adding a new flag swap_inprogress to indicate whether
>> the swap is ongoing.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>
> Indeed, commit 661d4f55a794 ("sbitmap: remove swap_lock") causes this
> issue, and ->cleared and ->word should have been checked & updated
> atomically.
>
>> Signed-off-by: Yang Yang <[email protected]>
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 22 ++++++++++++++++++++--
>> 2 files changed, 25 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
>> + */
>> + atomic_t swap_inprogress;
>> } ____cacheline_aligned_in_smp;
>>
>> /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..d4bb258fe8b0 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>> */
>> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> {
>> - unsigned long mask;
>> + unsigned long mask, flags;
>> + int zero = 0;
>>
>> - if (!READ_ONCE(map->cleared))
>> + if (!READ_ONCE(map->cleared)) {
>> + if (atomic_read(&map->swap_inprogress))
>> + goto out_wait;
>> return false;
>> + }
>> +
>> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
>> + goto out_wait;
>> +
>> + local_irq_save(flags);
>>
>> /*
>> * First get a stable cleared mask, setting the old mask to 0.
>> @@ -77,6 +86,15 @@ 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));
>> +
>> + atomic_set(&map->swap_inprogress, 0);
>> + smp_mb__after_atomic();
>> + local_irq_restore(flags);
>> + return true;
>> +
>> +out_wait:
>> + while (atomic_read(&map->swap_inprogress))
>> + ;
>> return true;
>> }
>
> IMO, the above fix looks a bit complicated, and I'd suggest to revert
> 661d4f55a794 ("sbitmap: remove swap_lock") if no clean & simple solution
> can be figured out.
Thanks. I will send a patch to revert that change.
>
> Thanks,
> Ming
>