On 2024/7/2 11:34, Ming Lei wrote:
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);
Sure, this looks better.
+ 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.
Yes. Thank you for the suggestion. I will include the following
descriptions in the next version.
/*
* 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 dead loop, we need to take
* wrap & alloc_hint into account. Without this, a soft
* lockup was detected in our test environment.
*/
Thanks.
Thanks,
Ming
