Commit a5867a21 authored by Yadan Fan's avatar Yadan Fan Committed by Andrew Morton
Browse files

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: default avatarYadan Fan <ydfan@suse.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 5631da56

mm/mempool.c

+33 −1
Original line number Diff line number Diff line
@@ -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);

			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);