@@ -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;
/**
@@ -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);
@@ -496,7 +518,7 @@ 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;
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> --- Changes from v7: - Eliminate the local variable 'ret' by suggestion - Simplify and optimize the if (READ_ONCE() ...) by suggestion Changes from v6: - Use guard() for locking by suggestion - Modify comments by suggestion Changes from v5: - Modify commit message - Change the fixes tag Changes from v4: - Add some comments according to suggestion 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 | 36 +++++++++++++++++++++++++++++------- 2 files changed, 34 insertions(+), 7 deletions(-)