Re: [PATCH v7] sbitmap: fix io hung due to race on sbitmap_word::cleared

From: YangYang
Date: Mon Jul 15 2024 - 23:48:26 EST


On 2024/7/16 1:26, Bart Van Assche wrote:
On 7/15/24 2:11 AM, Yang Yang wrote:
+
+    /**
+     * @swap_lock: serializes simultaneous updates of ->word and ->cleared
+     */
+    spinlock_t swap_lock;
  } ____cacheline_aligned_in_smp;

Thank you for having updated this comment.

-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;
+    bool ret = false;
-    if (!READ_ONCE(map->cleared))
-        return false;
+    guard(spinlock_irqsave)(&map->swap_lock);
+
+    if (!map->cleared) {
+        if (depth > 0) {
+            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);
+
+            if ((READ_ONCE(map->word) & word_mask) == word_mask)
+                ret = false;
+            else
+                ret = true;
+        }
+
+        return ret;
+    }

Now that guard()() is being used, the local variable 'ret' can be eliminated. The if (READ_ONCE() ...) statement can be changed into the
following: return (READ_ONCE(map->word) & word_mask) != word_mask;
and "return ret;" can be changed into "return false;". Additionally,
the indentation depth can be reduced by changing "if (depth > 0) {" ...
into "if (depth == 0) return false;".

This looks much better, I will make these changes in the next version.

Thanks.


Otherwise this patch looks good to me.

Thanks,

Bart.