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

Ming Lei
2024-06-06T03:12:50Z

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 <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):

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