[RFC PATCH v4 1/3] mm/zswap: release retired pools via queue_rcu_work() instead of synchronize_rcu()

[Jianyue Wu <wujianyue000@gmail.com>]

When a pool's last reference is dropped, __zswap_pool_empty() removes it
from the pool list and schedules __zswap_pool_release(), which calls
synchronize_rcu() to wait for readers before tearing the pool down.

synchronize_rcu() is a synchronous, potentially long wait.  Replace it
with queue_rcu_work(): __zswap_pool_empty() hands the pool to
queue_rcu_work(), which waits for a grace period asynchronously and then
runs __zswap_pool_release() from a worker for the sleepable teardown
(__zswap_pool_empty() can run in atomic context and must not block).
The grace-period guarantee is unchanged; the retirement path just no
longer blocks on it.

Suggested-by: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Jianyue Wu <wujianyue000@gmail.com>
---
 mm/zswap.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e..0bb30e58950a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -155,7 +155,7 @@ struct zswap_pool {
 	struct crypto_acomp_ctx __percpu *acomp_ctx;
 	struct percpu_ref ref;
 	struct list_head list;
-	struct work_struct release_work;
+	struct rcu_work release_work;
 	struct hlist_node node;
 	char tfm_name[CRYPTO_MAX_ALG_NAME];
 };
@@ -379,10 +379,8 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
 
 static void __zswap_pool_release(struct work_struct *work)
 {
-	struct zswap_pool *pool = container_of(work, typeof(*pool),
-						release_work);
-
-	synchronize_rcu();
+	struct zswap_pool *pool = container_of(to_rcu_work(work),
+					       typeof(*pool), release_work);
 
 	/* nobody should have been able to get a ref... */
 	WARN_ON(!percpu_ref_is_zero(&pool->ref));
@@ -406,8 +404,8 @@ static void __zswap_pool_empty(struct percpu_ref *ref)
 
 	list_del_rcu(&pool->list);
 
-	INIT_WORK(&pool->release_work, __zswap_pool_release);
-	schedule_work(&pool->release_work);
+	INIT_RCU_WORK(&pool->release_work, __zswap_pool_release);
+	queue_rcu_work(system_percpu_wq, &pool->release_work);
 
 	spin_unlock_bh(&zswap_pools_lock);
 }
-- 
2.43.0