Re: [PATCH v2] sbitmap: fix io hung due to race on sbitmap_word::cleared

YangYang
2024-06-06T08:55:24Z

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 <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.
>>>
>>> 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,
>>
>>
>> .
>>
>