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 <yang.yang@xxxxxxxx>
> ---
> 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
