mm: mempool: fix wake-up edge case bug for zero-minimum pools

The mempool wake-up path has a edge case bug that affects pools created
with min_nr=0.  When a thread blocks waiting for memory from an empty pool
(curr_nr == 0), subsequent mempool_free() calls fail to wake the waiting
thread because the condition "curr_nr < min_nr" evaluates to "0 < 0" which
is false, this can cause threads to sleep indefinitely according to the
code logic.

There is at least 2 places where the mempool created with min_nr=0:

1. lib/btree.c:191: mempool_create(0, btree_alloc, btree_free, NULL)
2. drivers/md/dm-verity-fec.c:791:
 mempool_init_slab_pool(&f->extra_pool, 0, f->cache)

Add an explicit check in mempool_free() to handle the min_nr=0 case: when
the pool has zero minimum reserves, is currently empty, and has active
waiters, allocate the element then wake up the sleeper.

Link: https://lkml.kernel.org/r/f28a81ba-615c-481e-86fb-c0bf4115ec89@suse.com
Signed-off-by: Yadan Fan <ydfan@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mempool.c b/mm/mempool.c
index 3223337135d0a4a75afbd0ede7a5f38b0d140691..204a216b6418fe260cf9d152c9920727f61d307c 100644 (file)
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -540,11 +540,43 @@ void mempool_free(void *element, mempool_t *pool)
                if (likely(pool->curr_nr < pool->min_nr)) {
                        add_element(pool, element);
                        spin_unlock_irqrestore(&pool->lock, flags);
-                       wake_up(&pool->wait);
+                       if (wq_has_sleeper(&pool->wait))
+                               wake_up(&pool->wait);
                        return;
                }
                spin_unlock_irqrestore(&pool->lock, flags);
        }
+
+       /*
+        * Handle the min_nr = 0 edge case:
+        *
+        * For zero-minimum pools, curr_nr < min_nr (0 < 0) never succeeds,
+        * so waiters sleeping on pool->wait would never be woken by the
+        * wake-up path of previous test. This explicit check ensures the
+        * allocation of element when both min_nr and curr_nr are 0, and
+        * any active waiters are properly awakened.
+        *
+        * Inline the same logic as previous test, add_element() cannot be
+        * directly used here since it has BUG_ON to deny if min_nr equals
+        * curr_nr, so here picked rest of add_element() to use without
+        * BUG_ON check.
+        */
+       if (unlikely(pool->min_nr == 0 &&
+                    READ_ONCE(pool->curr_nr) == 0)) {
+               spin_lock_irqsave(&pool->lock, flags);
+               if (likely(pool->curr_nr == 0)) {
+                       /* Inline the logic of add_element() */
+                       poison_element(pool, element);
+                       if (kasan_poison_element(pool, element))
+                               pool->elements[pool->curr_nr++] = element;
+                       spin_unlock_irqrestore(&pool->lock, flags);
+                       if (wq_has_sleeper(&pool->wait))
+                               wake_up(&pool->wait);
+                       return;
+               }
+               spin_unlock_irqrestore(&pool->lock, flags);
+       }
+
        pool->free(element, pool->pool_data);
 }
 EXPORT_SYMBOL(mempool_free);