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 <[email protected]>
---
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
On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>
> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
> remove swap_lock"), which causes this issue.
I'd suggest to add the following words in commit log:
Check on ->cleared and update on both ->cleared and ->word need to be
done atomically, and using spinlock could be the simplest solution.
Otherwise, the patch looks fine for me.
Thanks,
Hi,
在 2024/06/04 11:25, Ming Lei 写道:
> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>
>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>> remove swap_lock"), which causes this issue.
>
> I'd suggest to add the following words in commit log:
>
> Check on ->cleared and update on both ->cleared and ->word need to be
> done atomically, and using spinlock could be the simplest solution.
>
> Otherwise, the patch looks fine for me.
Maybe I'm noob, but I'm confused how can this fix the problem, looks
like the race condition doesn't change.
In sbitmap_find_bit_in_word:
1) __sbitmap_get_word read word;
2) sbitmap_deferred_clear clear cleared;
3) sbitmap_deferred_clear update word;
2) and 3) are done atomically while 1) can still concurrent with 3):
t1:
sbitmap_find_bit_in_word
__sbitmap_get_word
-> read old word, return -1
t2:
sbitmap_find_bit_in_word
__sbitmap_get_word
-> read old word, return -1
sbitmap_deferred_clear
-> clear cleared and update word
sbitmap_deferred_clear
-> cleared is cleared, fail
BYW, I still think it's fine to fix this problem by trying the
__sbitmap_get_word() at least one more time if __sbitmap_get_word()
failed.
Thanks,
Kuai
>
> Thanks,
>
>
> .
>
On 2024/6/4 11:25, Ming Lei wrote:
> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>
>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>> remove swap_lock"), which causes this issue.
>
> I'd suggest to add the following words in commit log:
>
> Check on ->cleared and update on both ->cleared and ->word need to be
> done atomically, and using spinlock could be the simplest solution.
Thank you. I will handle this in V3.
>
> Otherwise, the patch looks fine for me.
>
> Thanks,
>
On 2024/6/4 14:12, Yu Kuai wrote:
> Hi,
>
> 在 2024/06/04 11:25, Ming Lei 写道:
>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>
>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>> remove swap_lock"), which causes this issue.
>>
>> I'd suggest to add the following words in commit log:
>>
>> Check on ->cleared and update on both ->cleared and ->word need to be
>> done atomically, and using spinlock could be the simplest solution.
>>
>> Otherwise, the patch looks fine for me.
>
> Maybe I'm noob, but I'm confused how can this fix the problem, looks
> like the race condition doesn't change.
>
> In sbitmap_find_bit_in_word:
>
> 1) __sbitmap_get_word read word;
> 2) sbitmap_deferred_clear clear cleared;
> 3) sbitmap_deferred_clear update word;
>
> 2) and 3) are done atomically while 1) can still concurrent with 3):
>
> t1:
> sbitmap_find_bit_in_word
> __sbitmap_get_word
> -> read old word, return -1 > t2:
> sbitmap_find_bit_in_word
> __sbitmap_get_word
> -> read old word, return -1
> sbitmap_deferred_clear
> -> clear cleared and update word
> sbitmap_deferred_clear
> -> cleared is cleared, fail
Yes, you are right, this patch cannot fix this issue.
>
> BYW, I still think it's fine to fix this problem by trying the
> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
> failed.
Err, after trying one more time __sbitmap_get_word() may still fail.
>
> Thanks,
> Kuai
>
>>
>> Thanks,
>>
>>
>> .
>>
>
On Tue, Jun 04, 2024 at 02:12:22PM +0800, Yu Kuai wrote:
> Hi,
>
> 在 2024/06/04 11:25, Ming Lei 写道:
> > On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
> > >
> > > To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
> > > remove swap_lock"), which causes this issue.
> >
> > I'd suggest to add the following words in commit log:
> >
> > Check on ->cleared and update on both ->cleared and ->word need to be
> > done atomically, and using spinlock could be the simplest solution.
> >
> > Otherwise, the patch looks fine for me.
>
> Maybe I'm noob, but I'm confused how can this fix the problem, looks
> like the race condition doesn't change.
>
> In sbitmap_find_bit_in_word:
>
> 1) __sbitmap_get_word read word;
> 2) sbitmap_deferred_clear clear cleared;
> 3) sbitmap_deferred_clear update word;
>
> 2) and 3) are done atomically while 1) can still concurrent with 3):
After 1) fails, sbitmap_deferred_clear() is called with spinlock,
then it is pretty easy to solve the race, such as, the following patch
against the revert patch.
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index dee02a0266a6..c015ecd8e10e 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -63,13 +63,15 @@ 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;
+ bool ret;
spin_lock_irqsave(&map->swap_lock, flags);
- if (!map->cleared)
+ if (!map->cleared) {
+ ret = !!map->word;
goto out_unlock;
+ }
/*
* First get a stable cleared mask, setting the old mask to 0.
Thanks,
Ming
On 2024/6/6 11:12, Ming Lei wrote:
> On Tue, Jun 04, 2024 at 02:12:22PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2024/06/04 11:25, Ming Lei 写道:
>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>>
>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>> remove swap_lock"), which causes this issue.
>>>
>>> I'd suggest to add the following words in commit log:
>>>
>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>> done atomically, and using spinlock could be the simplest solution.
>>>
>>> Otherwise, the patch looks fine for me.
>>
>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>> like the race condition doesn't change.
>>
>> In sbitmap_find_bit_in_word:
>>
>> 1) __sbitmap_get_word read word;
>> 2) sbitmap_deferred_clear clear cleared;
>> 3) sbitmap_deferred_clear update word;
>>
>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>
> After 1) fails, sbitmap_deferred_clear() is called with spinlock,
> then it is pretty easy to solve the race, such as, the following patch
> against the revert patch.
>
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index dee02a0266a6..c015ecd8e10e 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -63,13 +63,15 @@ 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;
> + bool ret;
>
> spin_lock_irqsave(&map->swap_lock, flags);
>
> - if (!map->cleared)
> + if (!map->cleared) {
> + ret = !!map->word;
After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word
may be 0 if all requests have completed, or not 0 if some requests are
still in flight. Therefore, using !!map->word to determine the
availability of free tags is inaccurate.
Thanks
> goto out_unlock;
> + }
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
>
>
> Thanks,
> Ming
>
On 2024/6/4 14:12, Yu Kuai wrote:
> Hi,
>
> 在 2024/06/04 11:25, Ming Lei 写道:
>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>
>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>> remove swap_lock"), which causes this issue.
>>
>> I'd suggest to add the following words in commit log:
>>
>> Check on ->cleared and update on both ->cleared and ->word need to be
>> done atomically, and using spinlock could be the simplest solution.
>>
>> Otherwise, the patch looks fine for me.
>
> Maybe I'm noob, but I'm confused how can this fix the problem, looks
> like the race condition doesn't change.
>
> In sbitmap_find_bit_in_word:
>
> 1) __sbitmap_get_word read word;
> 2) sbitmap_deferred_clear clear cleared;
> 3) sbitmap_deferred_clear update word;
>
> 2) and 3) are done atomically while 1) can still concurrent with 3):
>
> t1:
> sbitmap_find_bit_in_word
> __sbitmap_get_word
> -> read old word, return -1
> t2:
> sbitmap_find_bit_in_word
> __sbitmap_get_word
> -> read old word, return -1
> sbitmap_deferred_clear
> -> clear cleared and update word
> sbitmap_deferred_clear
> -> cleared is cleared, fail
>
> BYW, I still think it's fine to fix this problem by trying the
> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
> failed.
How about this one:
1. Add extra check in sbitmap_find_bit_in_word() referenced from
Yu kuai's suggestion.
2. Change from atomic_long_andnot to atomic_long_fetch_andnot_release
---
include/linux/sbitmap.h | 5 +++++
lib/sbitmap.c | 23 ++++++++++++++++++-----
2 files changed, 23 insertions(+), 5 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..63dadf91e40b 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.
@@ -75,9 +79,12 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
/*
* Now clear the masked bits in our free word
*/
- atomic_long_andnot(mask, (atomic_long_t *)&map->word);
+ atomic_long_fetch_andnot_release(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);
@@ -175,11 +186,13 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
int nr;
do {
+ unsigned long cleared = READ_ONCE(map->cleared);
+
nr = __sbitmap_get_word(&map->word, depth,
alloc_hint, wrap);
if (nr != -1)
break;
- if (!sbitmap_deferred_clear(map))
+ if (!sbitmap_deferred_clear(map) && !cleared)
break;
} while (1);
--
2.34.1
>
> Thanks,
> Kuai
>
>>
>> Thanks,
>>
>>
>> .
>>
>
On Thu, Jun 06, 2024 at 03:21:38PM +0800, YangYang wrote:
> On 2024/6/6 11:12, Ming Lei wrote:
> > On Tue, Jun 04, 2024 at 02:12:22PM +0800, Yu Kuai wrote:
> > > Hi,
> > >
> > > 在 2024/06/04 11:25, Ming Lei 写道:
> > > > On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
> > > > >
> > > > > To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
> > > > > remove swap_lock"), which causes this issue.
> > > >
> > > > I'd suggest to add the following words in commit log:
> > > >
> > > > Check on ->cleared and update on both ->cleared and ->word need to be
> > > > done atomically, and using spinlock could be the simplest solution.
> > > >
> > > > Otherwise, the patch looks fine for me.
> > >
> > > Maybe I'm noob, but I'm confused how can this fix the problem, looks
> > > like the race condition doesn't change.
> > >
> > > In sbitmap_find_bit_in_word:
> > >
> > > 1) __sbitmap_get_word read word;
> > > 2) sbitmap_deferred_clear clear cleared;
> > > 3) sbitmap_deferred_clear update word;
> > >
> > > 2) and 3) are done atomically while 1) can still concurrent with 3):
> >
> > After 1) fails, sbitmap_deferred_clear() is called with spinlock,
> > then it is pretty easy to solve the race, such as, the following patch
> > against the revert patch.
> >
> >
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index dee02a0266a6..c015ecd8e10e 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -63,13 +63,15 @@ 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;
> > + bool ret;
> > spin_lock_irqsave(&map->swap_lock, flags);
> > - if (!map->cleared)
> > + if (!map->cleared) {
> > + ret = !!map->word;
>
> After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word
> may be 0 if all requests have completed, or not 0 if some requests are
> still in flight.
setting ->word is lockless, but zeroing ->word is serialized with ->swap_lock.
> Therefore, using !!map->word to determine the
> availability of free tags is inaccurate.
The check should be changed to decide if any free bit is available in
map->word instead of !!map->word, and 'shift' need to be passed in
sbitmap_deferred_clear().
Just be curious, do you have reproducer for this issue?
Thanks,
Ming
On 2024/6/6 17:02, Ming Lei wrote:
> On Thu, Jun 06, 2024 at 03:21:38PM +0800, YangYang wrote:
>> On 2024/6/6 11:12, Ming Lei wrote:
>>> On Tue, Jun 04, 2024 at 02:12:22PM +0800, Yu Kuai wrote:
>>>> Hi,
>>>>
>>>> 在 2024/06/04 11:25, Ming Lei 写道:
>>>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>>>>
>>>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>>>> remove swap_lock"), which causes this issue.
>>>>>
>>>>> I'd suggest to add the following words in commit log:
>>>>>
>>>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>>>> done atomically, and using spinlock could be the simplest solution.
>>>>>
>>>>> Otherwise, the patch looks fine for me.
>>>>
>>>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>>>> like the race condition doesn't change.
>>>>
>>>> In sbitmap_find_bit_in_word:
>>>>
>>>> 1) __sbitmap_get_word read word;
>>>> 2) sbitmap_deferred_clear clear cleared;
>>>> 3) sbitmap_deferred_clear update word;
>>>>
>>>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>>
>>> After 1) fails, sbitmap_deferred_clear() is called with spinlock,
>>> then it is pretty easy to solve the race, such as, the following patch
>>> against the revert patch.
>>>
>>>
>>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>>> index dee02a0266a6..c015ecd8e10e 100644
>>> --- a/lib/sbitmap.c
>>> +++ b/lib/sbitmap.c
>>> @@ -63,13 +63,15 @@ 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;
>>> + bool ret;
>>> spin_lock_irqsave(&map->swap_lock, flags);
>>> - if (!map->cleared)
>>> + if (!map->cleared) {
>>> + ret = !!map->word;
>>
>> After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word
>> may be 0 if all requests have completed, or not 0 if some requests are
>> still in flight.
>
> setting ->word is lockless, but zeroing ->word is serialized with ->swap_lock.
>
>> Therefore, using !!map->word to determine the
>> availability of free tags is inaccurate.
>
> The check should be changed to decide if any free bit is available in
> map->word instead of !!map->word, and 'shift' need to be passed in
> sbitmap_deferred_clear().
>
> Just be curious, do you have reproducer for this issue?
It's hard to reproduce, but it's reproducible. We've encountered this
issue several times.
Thanks
>
> Thanks,
> Ming
>
On Thu, Jun 06, 2024 at 04:55:17PM +0800, YangYang wrote:
> On 2024/6/4 14:12, Yu Kuai wrote:
> > Hi,
> >
> > 在 2024/06/04 11:25, Ming Lei 写道:
> > > On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
> > > >
> > > > To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
> > > > remove swap_lock"), which causes this issue.
> > >
> > > I'd suggest to add the following words in commit log:
> > >
> > > Check on ->cleared and update on both ->cleared and ->word need to be
> > > done atomically, and using spinlock could be the simplest solution.
> > >
> > > Otherwise, the patch looks fine for me.
> >
> > Maybe I'm noob, but I'm confused how can this fix the problem, looks
> > like the race condition doesn't change.
> >
> > In sbitmap_find_bit_in_word:
> >
> > 1) __sbitmap_get_word read word;
> > 2) sbitmap_deferred_clear clear cleared;
> > 3) sbitmap_deferred_clear update word;
> >
> > 2) and 3) are done atomically while 1) can still concurrent with 3):
> >
> > t1:
> > sbitmap_find_bit_in_word
> > __sbitmap_get_word
> > -> read old word, return -1
> > t2:
> > sbitmap_find_bit_in_word
> > __sbitmap_get_word
> > -> read old word, return -1
> > sbitmap_deferred_clear
> > -> clear cleared and update word
> > sbitmap_deferred_clear
> > -> cleared is cleared, fail
> >
> > BYW, I still think it's fine to fix this problem by trying the
> > __sbitmap_get_word() at least one more time if __sbitmap_get_word()
> > failed.
>
> How about this one:
> 1. Add extra check in sbitmap_find_bit_in_word() referenced from
> Yu kuai's suggestion.
> 2. Change from atomic_long_andnot to atomic_long_fetch_andnot_release
>
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 23 ++++++++++++++++++-----
> 2 files changed, 23 insertions(+), 5 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..63dadf91e40b 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.
> @@ -75,9 +79,12 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> /*
> * Now clear the masked bits in our free word
> */
> - atomic_long_andnot(mask, (atomic_long_t *)&map->word);
> + atomic_long_fetch_andnot_release(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);
> @@ -175,11 +186,13 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
> int nr;
>
> do {
> + unsigned long cleared = READ_ONCE(map->cleared);
> +
->cleared is stored in standalone cacheline, the above line adds one extra l1
load, so I still prefer to check ->word & ->cleared in sbitmap_deferred_clear().
Thanks,
Ming
On 2024/6/6 23:48, Ming Lei wrote:
> On Thu, Jun 06, 2024 at 04:55:17PM +0800, YangYang wrote:
>> On 2024/6/4 14:12, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2024/06/04 11:25, Ming Lei 写道:
>>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>>>
>>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>>> remove swap_lock"), which causes this issue.
>>>>
>>>> I'd suggest to add the following words in commit log:
>>>>
>>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>>> done atomically, and using spinlock could be the simplest solution.
>>>>
>>>> Otherwise, the patch looks fine for me.
>>>
>>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>>> like the race condition doesn't change.
>>>
>>> In sbitmap_find_bit_in_word:
>>>
>>> 1) __sbitmap_get_word read word;
>>> 2) sbitmap_deferred_clear clear cleared;
>>> 3) sbitmap_deferred_clear update word;
>>>
>>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>>
>>> t1:
>>> sbitmap_find_bit_in_word
>>> __sbitmap_get_word
>>> -> read old word, return -1
>>> t2:
>>> sbitmap_find_bit_in_word
>>> __sbitmap_get_word
>>> -> read old word, return -1
>>> sbitmap_deferred_clear
>>> -> clear cleared and update word
>>> sbitmap_deferred_clear
>>> -> cleared is cleared, fail
>>>
>>> BYW, I still think it's fine to fix this problem by trying the
>>> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
>>> failed.
>>
>> How about this one:
>> 1. Add extra check in sbitmap_find_bit_in_word() referenced from
>> Yu kuai's suggestion.
>> 2. Change from atomic_long_andnot to atomic_long_fetch_andnot_release
>>
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 23 ++++++++++++++++++-----
>> 2 files changed, 23 insertions(+), 5 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..63dadf91e40b 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.
>> @@ -75,9 +79,12 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> /*
>> * Now clear the masked bits in our free word
>> */
>> - atomic_long_andnot(mask, (atomic_long_t *)&map->word);
>> + atomic_long_fetch_andnot_release(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);
>> @@ -175,11 +186,13 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
>> int nr;
>>
>> do {
>> + unsigned long cleared = READ_ONCE(map->cleared);
>> +
>
> ->cleared is stored in standalone cacheline, the above line adds one extra l1
> load, so I still prefer to check ->word & ->cleared in sbitmap_deferred_clear().
Yes, it's very reasonable.
I have sent V3, which does the check in sbitmap_deferred_clear()
referenced from your suggestion.
+ if (!map->cleared) {
+ ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
+ goto out_unlock;
+ }
https://lore.kernel.org/linux-block/[email protected]/T/#u
Thanks.
On 6/4/24 08:03, YangYang wrote:
> On 2024/6/4 14:12, Yu Kuai wrote:
>> Hi,
>>
>> 在 2024/06/04 11:25, Ming Lei 写道:
>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>>
>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>> remove swap_lock"), which causes this issue.
>>>
>>> I'd suggest to add the following words in commit log:
>>>
>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>> done atomically, and using spinlock could be the simplest solution.
>>>
>>> Otherwise, the patch looks fine for me.
>>
>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>> like the race condition doesn't change.
>>
>> In sbitmap_find_bit_in_word:
>>
>> 1) __sbitmap_get_word read word;
>> 2) sbitmap_deferred_clear clear cleared;
>> 3) sbitmap_deferred_clear update word;
>>
>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>
>> t1:
>> sbitmap_find_bit_in_word
>> __sbitmap_get_word
>> -> read old word, return -1 > t2:
>> sbitmap_find_bit_in_word
>> __sbitmap_get_word
>> -> read old word, return -1
>> sbitmap_deferred_clear
>> -> clear cleared and update word
>> sbitmap_deferred_clear
>> -> cleared is cleared, fail
>
> Yes, you are right, this patch cannot fix this issue.
One other alternative is to kill ->cleared. It's not
immediately clear how important it is. Do we have any
numbers?
>> BYW, I still think it's fine to fix this problem by trying the
>> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
>> failed.
>
> Err, after trying one more time __sbitmap_get_word() may still fail.
--
Pavel Begunkov
On 2024/6/7 20:59, Pavel Begunkov wrote:
> On 6/4/24 08:03, YangYang wrote:
>> On 2024/6/4 14:12, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2024/06/04 11:25, Ming Lei 写道:
>>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
>>>>>
>>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>>> remove swap_lock"), which causes this issue.
>>>>
>>>> I'd suggest to add the following words in commit log:
>>>>
>>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>>> done atomically, and using spinlock could be the simplest solution.
>>>>
>>>> Otherwise, the patch looks fine for me.
>>>
>>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>>> like the race condition doesn't change.
>>>
>>> In sbitmap_find_bit_in_word:
>>>
>>> 1) __sbitmap_get_word read word;
>>> 2) sbitmap_deferred_clear clear cleared;
>>> 3) sbitmap_deferred_clear update word;
>>>
>>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>>
>>> t1:
>>> sbitmap_find_bit_in_word
>>> __sbitmap_get_word
>>> -> read old word, return -1 > t2:
>>> sbitmap_find_bit_in_word
>>> __sbitmap_get_word
>>> -> read old word, return -1
>>> sbitmap_deferred_clear
>>> -> clear cleared and update word
>>> sbitmap_deferred_clear
>>> -> cleared is cleared, fail
>>
>> Yes, you are right, this patch cannot fix this issue.
>
> One other alternative is to kill ->cleared. It's not
> immediately clear how important it is. Do we have any
> numbers?
Sorry, I can't get it. Are you suggesting to remove ->cleared from
struct sbitmap_word entirely?
Thanks.
>
>>> BYW, I still think it's fine to fix this problem by trying the
>>> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
>>> failed.
>>
>> Err, after trying one more time __sbitmap_get_word() may still fail.
>
On Fri, Jun 07, 2024 at 01:59:07PM +0100, Pavel Begunkov wrote:
> On 6/4/24 08:03, YangYang wrote:
> > On 2024/6/4 14:12, Yu Kuai wrote:
> > > Hi,
> > >
> > > 在 2024/06/04 11:25, Ming Lei 写道:
> > > > On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <[email protected]> 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.
> > > > >
> > > > > To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
> > > > > remove swap_lock"), which causes this issue.
> > > >
> > > > I'd suggest to add the following words in commit log:
> > > >
> > > > Check on ->cleared and update on both ->cleared and ->word need to be
> > > > done atomically, and using spinlock could be the simplest solution.
> > > >
> > > > Otherwise, the patch looks fine for me.
> > >
> > > Maybe I'm noob, but I'm confused how can this fix the problem, looks
> > > like the race condition doesn't change.
> > >
> > > In sbitmap_find_bit_in_word:
> > >
> > > 1) __sbitmap_get_word read word;
> > > 2) sbitmap_deferred_clear clear cleared;
> > > 3) sbitmap_deferred_clear update word;
> > >
> > > 2) and 3) are done atomically while 1) can still concurrent with 3):
> > >
> > > t1:
> > > sbitmap_find_bit_in_word
> > > __sbitmap_get_word
> > > -> read old word, return -1 > t2:
> > > sbitmap_find_bit_in_word
> > > __sbitmap_get_word
> > > -> read old word, return -1
> > > sbitmap_deferred_clear
> > > -> clear cleared and update word
> > > sbitmap_deferred_clear
> > > -> cleared is cleared, fail
> >
> > Yes, you are right, this patch cannot fix this issue.
>
> One other alternative is to kill ->cleared. It's not
> immediately clear how important it is. Do we have any
> numbers?
Please see commit ea86ea2cdced ("sbitmap: ammortize cost of clearing bits").
```
In a threaded poll test case, half the overhead of getting and clearing
tags is removed with this change.
```
Thanks,
Ming