On Mon, Jun 17, 2024 at 10:45:51AM +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.
>
> This patch achieves two purposes:
> 1. Check on ->cleared and update on both ->cleared and ->word need to
> be done atomically, and using spinlock could be the simplest solution.
> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> may cause potential race.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
>
> ---
> Changes from v3:
> - Add more arguments to sbitmap_deferred_clear(), for those who
> don't care about the return value, just pass 0
> - Consider the situation when using sbitmap_get_shallow()
> - Consider the situation when ->round_robin is true
> - Modify commit message
> Changes from v2:
> - Modify commit message by suggestion
> - Add extra check in sbitmap_deferred_clear() by suggestion
> Changes from v1:
> - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++---------
> 2 files changed, 41 insertions(+), 9 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..9bd85a9b74b9 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> /*
> * See if we have deferred clears that we can batch move
> */
> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> -{
> - unsigned long mask;
> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
> + unsigned int depth, unsigned int alloc_hint, bool wrap)
> +{
> + unsigned long mask, flags, word_mask;
> + bool ret = false;
> +
> + spin_lock_irqsave(&map->swap_lock, flags);
> +
> + if (!map->cleared) {
> + if (depth > 0) {
> +#if BITS_PER_LONG == 64
> + word_mask = U64_MAX >> (BITS_PER_LONG - depth);
> +#else
> + word_mask = U32_MAX >> (BITS_PER_LONG - depth);
> +#endif
Can we avoid the above conditional compiling by the following way?
word_mask = (~0UL) >> (BITS_PER_LONG - depth);
> + if (!wrap && alloc_hint)
> + word_mask &= ~((1UL << alloc_hint) - 1);
The current behavior is to always retry after moving ->cleared to
word, and we change it to retry in case of any free bits. So here
is it for avoiding dead loop by taking wrap & alloc_hint into
account? If yes, this way looks fine, but I'd suggest to document
this change.
Thanks,
Ming
