linux-kernel - [PATCH mm-unstable hotfix] mm/zswap: fix zswap_pools_lock usages after changing to percpu

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Date: Wed, 28 Feb 2024 15:18:32 +0000
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: akpm@...ux-foundation.org
Cc: hannes@...xchg.org,
	yosryahmed@...gle.com,
	nphamcs@...il.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Chengming Zhou <chengming.zhou@...ux.dev>
Subject: [PATCH mm-unstable hotfix] mm/zswap: fix zswap_pools_lock usages after changing to percpu_ref

Now the release of zswap pool is controlled by percpu_ref, its release
callback (__zswap_pool_empty()) will be called when percpu_ref hit 0.
But this release callback may potentially be called from RCU callback
context by percpu_ref_kill(), which maybe in the interrupt context.

So we need to use spin_lock_irqsave() and spin_unlock_irqrestore()
in the release callback: __zswap_pool_empty(). In other task context
places, spin_lock_irq() and spin_unlock_irq() are enough to avoid
potential deadlock.

This problem is introduced by the commit f3da427e82c4 ("mm/zswap: change
zswap_pool kref to percpu_ref"), which is in mm-unstable branch now.
It can be reproduced by testing kernel build in tmpfs with zswap and
CONFIG_LOCKDEP enabled, meanwhile changing the zswap compressor setting
dynamically.

Signed-off-by: Chengming Zhou <chengming.zhou@...ux.dev>
---
 mm/zswap.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 011e068eb355..894bd184f78e 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -456,10 +456,11 @@ static struct zswap_pool *zswap_pool_current(void);
 static void __zswap_pool_empty(struct percpu_ref *ref)
 {
 	struct zswap_pool *pool;
+	unsigned long flags;
 
 	pool = container_of(ref, typeof(*pool), ref);
 
-	spin_lock(&zswap_pools_lock);
+	spin_lock_irqsave(&zswap_pools_lock, flags);
 
 	WARN_ON(pool == zswap_pool_current());
 
@@ -468,7 +469,7 @@ static void __zswap_pool_empty(struct percpu_ref *ref)
 	INIT_WORK(&pool->release_work, __zswap_pool_release);
 	schedule_work(&pool->release_work);
 
-	spin_unlock(&zswap_pools_lock);
+	spin_unlock_irqrestore(&zswap_pools_lock, flags);
 }
 
 static int __must_check zswap_pool_get(struct zswap_pool *pool)
@@ -598,7 +599,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 		return -EINVAL;
 	}
 
-	spin_lock(&zswap_pools_lock);
+	spin_lock_irq(&zswap_pools_lock);
 
 	pool = zswap_pool_find_get(type, compressor);
 	if (pool) {
@@ -607,7 +608,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 		list_del_rcu(&pool->list);
 	}
 
-	spin_unlock(&zswap_pools_lock);
+	spin_unlock_irq(&zswap_pools_lock);
 
 	if (!pool)
 		pool = zswap_pool_create(type, compressor);
@@ -628,7 +629,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 	else
 		ret = -EINVAL;
 
-	spin_lock(&zswap_pools_lock);
+	spin_lock_irq(&zswap_pools_lock);
 
 	if (!ret) {
 		put_pool = zswap_pool_current();
@@ -643,7 +644,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 		put_pool = pool;
 	}
 
-	spin_unlock(&zswap_pools_lock);
+	spin_unlock_irq(&zswap_pools_lock);
 
 	if (!zswap_has_pool && !pool) {
 		/* if initial pool creation failed, and this pool creation also
-- 
2.40.1