[PATCH] mm, list_lru: refactor the locking code

[PATCH] mm, list_lru: refactor the locking code

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

Cocci is confused by the try lock then release RCU and return logic
here. So separate the try lock part out into a standalone helper. The
code is easier to follow too.

No feature change, fixes:

cocci warnings: (new ones prefixed by >>)
>> mm/list_lru.c:82:3-9: preceding lock on line 77
>> mm/list_lru.c:82:3-9: preceding lock on line 77
   mm/list_lru.c:82:3-9: preceding lock on line 75
   mm/list_lru.c:82:3-9: preceding lock on line 75

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/r/202505252043.pbT1tBHJ-lkp@intel.com/
Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/list_lru.c | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 490473af3122..ec48b5dadf51 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -60,30 +60,34 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
 	return &lru->node[nid].lru;
 }
 
+static inline bool lock_list_lru(struct list_lru_one *l, bool irq)
+{
+	if (irq)
+		spin_lock_irq(&l->lock);
+	else
+		spin_lock(&l->lock);
+	if (unlikely(READ_ONCE(l->nr_items) == LONG_MIN)) {
+		if (irq)
+			spin_unlock_irq(&l->lock);
+		else
+			spin_unlock(&l->lock);
+		return false;
+	}
+	return true;
+}
+
 static inline struct list_lru_one *
 lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
 		       bool irq, bool skip_empty)
 {
 	struct list_lru_one *l;
-	long nr_items;
 
 	rcu_read_lock();
 again:
 	l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
-	if (likely(l)) {
-		if (irq)
-			spin_lock_irq(&l->lock);
-		else
-			spin_lock(&l->lock);
-		nr_items = READ_ONCE(l->nr_items);
-		if (likely(nr_items != LONG_MIN)) {
-			rcu_read_unlock();
-			return l;
-		}
-		if (irq)
-			spin_unlock_irq(&l->lock);
-		else
-			spin_unlock(&l->lock);
+	if (likely(l) && lock_list_lru(l, irq)) {
+		rcu_read_unlock();
+		return l;
 	}
 	/*
 	 * Caller may simply bail out if raced with reparenting or
-- 
2.49.0

Re: [PATCH] mm, list_lru: refactor the locking code

[Qi Zheng]

On 5/27/25 2:06 AM, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Cocci is confused by the try lock then release RCU and return logic
> here. So separate the try lock part out into a standalone helper. The
> code is easier to follow too.
> 
> No feature change, fixes:
> 
> cocci warnings: (new ones prefixed by >>)
>>> mm/list_lru.c:82:3-9: preceding lock on line 77
>>> mm/list_lru.c:82:3-9: preceding lock on line 77
>     mm/list_lru.c:82:3-9: preceding lock on line 75
>     mm/list_lru.c:82:3-9: preceding lock on line 75
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Julia Lawall <julia.lawall@inria.fr>
> Closes: https://lore.kernel.org/r/202505252043.pbT1tBHJ-lkp@intel.com/
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>   mm/list_lru.c | 34 +++++++++++++++++++---------------
>   1 file changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 490473af3122..ec48b5dadf51 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -60,30 +60,34 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
>   	return &lru->node[nid].lru;
>   }
>   
> +static inline bool lock_list_lru(struct list_lru_one *l, bool irq)
> +{
> +	if (irq)
> +		spin_lock_irq(&l->lock);
> +	else
> +		spin_lock(&l->lock);
> +	if (unlikely(READ_ONCE(l->nr_items) == LONG_MIN)) {
> +		if (irq)
> +			spin_unlock_irq(&l->lock);
> +		else
> +			spin_unlock(&l->lock);
> +		return false;
> +	}
> +	return true;
> +}
> +
>   static inline struct list_lru_one *
>   lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
>   		       bool irq, bool skip_empty)
>   {
>   	struct list_lru_one *l;
> -	long nr_items;
>   
>   	rcu_read_lock();
>   again:
>   	l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
> -	if (likely(l)) {
> -		if (irq)
> -			spin_lock_irq(&l->lock);
> -		else
> -			spin_lock(&l->lock);
> -		nr_items = READ_ONCE(l->nr_items);
> -		if (likely(nr_items != LONG_MIN)) {
> -			rcu_read_unlock();
> -			return l;
> -		}
> -		if (irq)
> -			spin_unlock_irq(&l->lock);
> -		else
> -			spin_unlock(&l->lock);
> +	if (likely(l) && lock_list_lru(l, irq)) {
> +		rcu_read_unlock();
> +		return l;
>   	}
>   	/*
>   	 * Caller may simply bail out if raced with reparenting or

And the code readability has also been improved.

Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>

Thanks!

Re: [PATCH] mm, list_lru: refactor the locking code

[Muchun Song]

> On May 27, 2025, at 02:06, Kairui Song <ryncsn@gmail.com> wrote:
> 
> From: Kairui Song <kasong@tencent.com>
> 
> Cocci is confused by the try lock then release RCU and return logic
> here. So separate the try lock part out into a standalone helper. The
> code is easier to follow too.
> 
> No feature change, fixes:
> 
> cocci warnings: (new ones prefixed by >>)
>>> mm/list_lru.c:82:3-9: preceding lock on line 77
>>> mm/list_lru.c:82:3-9: preceding lock on line 77
>   mm/list_lru.c:82:3-9: preceding lock on line 75
>   mm/list_lru.c:82:3-9: preceding lock on line 75
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Julia Lawall <julia.lawall@inria.fr>
> Closes: https://lore.kernel.org/r/202505252043.pbT1tBHJ-lkp@intel.com/
> Signed-off-by: Kairui Song <kasong@tencent.com>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.

Re: [PATCH] mm, list_lru: refactor the locking code

[SeongJae Park]

Hi Kairui,

On Tue, 27 May 2025 02:06:38 +0800 Kairui Song <ryncsn@gmail.com> wrote:

> From: Kairui Song <kasong@tencent.com>
> 
> Cocci is confused by the try lock then release RCU and return logic
> here. So separate the try lock part out into a standalone helper. The
> code is easier to follow too.
> 
> No feature change, fixes:
> 
> cocci warnings: (new ones prefixed by >>)
> >> mm/list_lru.c:82:3-9: preceding lock on line 77
> >> mm/list_lru.c:82:3-9: preceding lock on line 77
>    mm/list_lru.c:82:3-9: preceding lock on line 75
>    mm/list_lru.c:82:3-9: preceding lock on line 75
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Julia Lawall <julia.lawall@inria.fr>
> Closes: https://lore.kernel.org/r/202505252043.pbT1tBHJ-lkp@intel.com/
> Signed-off-by: Kairui Song <kasong@tencent.com>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
>  mm/list_lru.c | 34 +++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 490473af3122..ec48b5dadf51 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -60,30 +60,34 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
>  	return &lru->node[nid].lru;
>  }
>  
> +static inline bool lock_list_lru(struct list_lru_one *l, bool irq)
> +{
> +	if (irq)
> +		spin_lock_irq(&l->lock);
> +	else
> +		spin_lock(&l->lock);
> +	if (unlikely(READ_ONCE(l->nr_items) == LONG_MIN)) {
> +		if (irq)
> +			spin_unlock_irq(&l->lock);
> +		else
> +			spin_unlock(&l->lock);
> +		return false;
> +	}

I'd prefer 'if (likely(...)) return true;' to reduce indentation and stop
wondering what goes to the likely case earlier.  But that's my personal
preferrence that shouldn't block this.

> +	return true;
> +}
> +
>  static inline struct list_lru_one *
>  lock_list_lru_of_memcg(struct list_lru *lru, int nid, struct mem_cgroup *memcg,
>  		       bool irq, bool skip_empty)
>  {
>  	struct list_lru_one *l;
> -	long nr_items;
>  
>  	rcu_read_lock();
>  again:
>  	l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
> -	if (likely(l)) {
> -		if (irq)
> -			spin_lock_irq(&l->lock);
> -		else
> -			spin_lock(&l->lock);
> -		nr_items = READ_ONCE(l->nr_items);
> -		if (likely(nr_items != LONG_MIN)) {
> -			rcu_read_unlock();
> -			return l;
> -		}
> -		if (irq)
> -			spin_unlock_irq(&l->lock);
> -		else
> -			spin_unlock(&l->lock);
> +	if (likely(l) && lock_list_lru(l, irq)) {
> +		rcu_read_unlock();
> +		return l;
>  	}

Much easier to read, indeed :)

>  	/*
>  	 * Caller may simply bail out if raced with reparenting or
> -- 
> 2.49.0


Thanks,
SJ

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