Re: [PATCH v7 1/4] lib/percpu-list: Per-cpu list with associated per-cpu locks

[Boqun Feng]

Hi Waiman,

On Tue, Apr 12, 2016 at 06:54:43PM -0400, Waiman Long wrote:
[...]
> +
> +/*
> + * Initialize the per-cpu list head
> + */
> +int init_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
> +{
> +	struct pcpu_list_head *pcpu_head = alloc_percpu(struct pcpu_list_head);
> +	int cpu;
> +
> +	if (!pcpu_head)
> +		return -ENOMEM;
> +
> +	for_each_possible_cpu(cpu) {
> +		struct pcpu_list_head *head = per_cpu_ptr(pcpu_head, cpu);
> +
> +		INIT_LIST_HEAD(&head->list);
> +		head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
> +		lockdep_set_class(&head->lock, &percpu_list_key);
> +	}
> +
> +	*ppcpu_head = pcpu_head;
> +	return 0;
> +}

The first time I looked at this patch, I had a hard time to figure out
which "struct pcpu_list_head" pointer is pointing to percpu data(the
pointer could be the parameter for per/this_cpu_ptr()), and which
pointer is pointing to actual structure. For example, 'pcpu_head' and
'head' above are different types of pointers.

So besides improving my code reading skills, I think the following patch
helps ;-) Also it can resolve several splats of sparse when running
'make C=1 lib/'.

Thoughts?

Regards,
Boqun

-------------------------------------------->8
From: Boqun Feng <boqun.feng@xxxxxxxxx>
Date: Wed, 13 Apr 2016 09:49:13 +0800
Subject: [PATCH] lib/percpu-list: Add __percpu modifier for parameters

Add __percpu modifier properly to help:

1.	Differ pointers to actual structures with those to percpu
	structures, which could improve readability.

2. 	Prevent sparse from complaining about "different address spaces"

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/percpu-list.h | 16 +++++++++-------
 lib/percpu-list.c           |  8 +++++---
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/include/linux/percpu-list.h b/include/linux/percpu-list.h
index ce8238a78198..4c8496004dc2 100644
--- a/include/linux/percpu-list.h
+++ b/include/linux/percpu-list.h
@@ -107,7 +107,8 @@ static inline void init_pcpu_list_node(struct pcpu_list_node *node)
 	node->lockptr = NULL;
 }
 
-static inline void free_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
+static inline void
+free_pcpu_list_head(struct pcpu_list_head __percpu **ppcpu_head)
 {
 	free_percpu(*ppcpu_head);
 	*ppcpu_head = NULL;
@@ -116,7 +117,7 @@ static inline void free_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
 /*
  * Check if all the per-cpu lists are empty
  */
-static inline bool pcpu_list_empty(struct pcpu_list_head *pcpu_head)
+static inline bool pcpu_list_empty(struct pcpu_list_head __percpu *pcpu_head)
 {
 	int cpu;
 
@@ -133,7 +134,8 @@ static inline bool pcpu_list_empty(struct pcpu_list_head *pcpu_head)
  * Return: true if the entry is found, false if all the lists exhausted
  */
 static __always_inline bool
-__pcpu_list_next_cpu(struct pcpu_list_head *head, struct pcpu_list_state *state)
+__pcpu_list_next_cpu(struct pcpu_list_head __percpu *head,
+		     struct pcpu_list_state *state)
 {
 	if (state->lock)
 		spin_unlock(state->lock);
@@ -170,7 +172,7 @@ next_cpu:
  *
  * Return: true if the next entry is found, false if all the entries iterated
  */
-static inline bool pcpu_list_iterate(struct pcpu_list_head *head,
+static inline bool pcpu_list_iterate(struct pcpu_list_head __percpu *head,
 				     struct pcpu_list_state *state)
 {
 	/*
@@ -198,7 +200,7 @@ static inline bool pcpu_list_iterate(struct pcpu_list_head *head,
  *
  * Return: true if the next entry is found, false if all the entries iterated
  */
-static inline bool pcpu_list_iterate_safe(struct pcpu_list_head *head,
+static inline bool pcpu_list_iterate_safe(struct pcpu_list_head __percpu *head,
 					  struct pcpu_list_state *state)
 {
 	/*
@@ -224,8 +226,8 @@ static inline bool pcpu_list_iterate_safe(struct pcpu_list_head *head,
 }
 
 extern void pcpu_list_add(struct pcpu_list_node *node,
-			  struct pcpu_list_head *head);
+			  struct pcpu_list_head __percpu *head);
 extern void pcpu_list_del(struct pcpu_list_node *node);
-extern int  init_pcpu_list_head(struct pcpu_list_head **ppcpu_head);
+extern int  init_pcpu_list_head(struct pcpu_list_head __percpu **ppcpu_head);
 
 #endif /* __LINUX_PERCPU_LIST_H */
diff --git a/lib/percpu-list.c b/lib/percpu-list.c
index 8a9600169966..ef2bcb8e5a1b 100644
--- a/lib/percpu-list.c
+++ b/lib/percpu-list.c
@@ -27,9 +27,10 @@ static struct lock_class_key percpu_list_key;
 /*
  * Initialize the per-cpu list head
  */
-int init_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
+int init_pcpu_list_head(struct pcpu_list_head __percpu **ppcpu_head)
 {
-	struct pcpu_list_head *pcpu_head = alloc_percpu(struct pcpu_list_head);
+	struct pcpu_list_head __percpu *pcpu_head =
+				alloc_percpu(struct pcpu_list_head);
 	int cpu;
 
 	if (!pcpu_head)
@@ -52,7 +53,8 @@ int init_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
  * function is called. However, deletion may be done by a different CPU.
  * So we still need to use a lock to protect the content of the list.
  */
-void pcpu_list_add(struct pcpu_list_node *node, struct pcpu_list_head *head)
+void pcpu_list_add(struct pcpu_list_node *node,
+		   struct pcpu_list_head __percpu *head)
 {
 	struct pcpu_list_head *myhead;
 
-- 
2.8.0