6.1-stable review patch. If anyone has any objections, please let me know.
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From: Yang Yang yang.yang@vivo.com
[ Upstream commit 72d04bdcf3f7d7e07d82f9757946f68802a7270a ]
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. 2. Add extra check in sbitmap_deferred_clear(), to identify whether ->word has free bits.
Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits") Signed-off-by: Yang Yang yang.yang@vivo.com Reviewed-by: Ming Lei ming.lei@redhat.com Reviewed-by: Bart Van Assche bvanassche@acm.org Link: https://lore.kernel.org/r/20240716082644.659566-1-yang.yang@vivo.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/sbitmap.h | 5 +++++ lib/sbitmap.c | 36 +++++++++++++++++++++++++++++------- 2 files changed, 34 insertions(+), 7 deletions(-)
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h index d662cf136021d..c09cdcc99471e 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: serializes simultaneous updates of ->word and ->cleared + */ + spinlock_t swap_lock; } ____cacheline_aligned_in_smp;
/** diff --git a/lib/sbitmap.c b/lib/sbitmap.c index a727d0b12763a..61075535a8073 100644 --- a/lib/sbitmap.c +++ b/lib/sbitmap.c @@ -60,12 +60,30 @@ 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) +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map, + unsigned int depth, unsigned int alloc_hint, bool wrap) { - unsigned long mask; + unsigned long mask, word_mask;
- if (!READ_ONCE(map->cleared)) - return false; + guard(spinlock_irqsave)(&map->swap_lock); + + if (!map->cleared) { + if (depth == 0) + return false; + + word_mask = (~0UL) >> (BITS_PER_LONG - depth); + /* + * The current behavior is to always retry after moving + * ->cleared to word, and we change it to retry in case + * of any free bits. To avoid an infinite loop, we need + * to take wrap & alloc_hint into account, otherwise a + * soft lockup may occur. + */ + if (!wrap && alloc_hint) + word_mask &= ~((1UL << alloc_hint) - 1); + + return (READ_ONCE(map->word) & word_mask) != word_mask; + }
/* * First get a stable cleared mask, setting the old mask to 0. @@ -85,6 +103,7 @@ 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 +135,9 @@ 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); @@ -126,7 +148,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth) unsigned int i;
for (i = 0; i < sb->map_nr; i++) - sbitmap_deferred_clear(&sb->map[i]); + sbitmap_deferred_clear(&sb->map[i], 0, 0, 0);
sb->depth = depth; sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word); @@ -179,7 +201,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map, alloc_hint, wrap); if (nr != -1) break; - if (!sbitmap_deferred_clear(map)) + if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap)) break; } while (1);
@@ -505,7 +527,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags, unsigned int map_depth = __map_depth(sb, index); unsigned long val;
- sbitmap_deferred_clear(map); + sbitmap_deferred_clear(map, 0, 0, 0); val = READ_ONCE(map->word); if (val == (1UL << (map_depth - 1)) - 1) goto next;