[PATCH v2] list: Add a macro to test if entry is pointing to the head

[PATCH v2] list: Add a macro to test if entry is pointing to the head

[Andy Shevchenko]

Add a macro to test if entry is pointing to the head of the list
which is useful in cases like:

  list_for_each_entry(pos, &head, member) {
    if (cond)
      break;
  }
  if (list_entry_is_head(pos, &head, member))
    return -ERRNO;

that allows to avoid additional variable to be added to track if loop
has not been stopped in the middle.

While here, convert list_for_each_entry*() family of macros to use a new one.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
v2: converted users inside list.h, dropped ambiguous description
 include/linux/list.h | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 796975c3c35c..89bdc92e75c3 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -609,6 +609,15 @@ static inline void list_splice_tail_init(struct list_head *list,
 	     pos != (head); \
 	     pos = n, n = pos->prev)
 
+/**
+ * list_entry_is_head - test if the entry points to the head of the list
+ * @pos:	the type * to cursor
+ * @head:	the head for your list.
+ * @member:	the name of the list_head within the struct.
+ */
+#define list_entry_is_head(pos, head, member)				\
+	(&pos->member == (head))
+
 /**
  * list_for_each_entry	-	iterate over list of given type
  * @pos:	the type * to use as a loop cursor.
@@ -617,7 +626,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_for_each_entry(pos, head, member)				\
 	for (pos = list_first_entry(head, typeof(*pos), member);	\
-	     &pos->member != (head);					\
+	     !list_entry_is_head(pos, head, member);			\
 	     pos = list_next_entry(pos, member))
 
 /**
@@ -628,7 +637,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_for_each_entry_reverse(pos, head, member)			\
 	for (pos = list_last_entry(head, typeof(*pos), member);		\
-	     &pos->member != (head); 					\
+	     !list_entry_is_head(pos, head, member); 			\
 	     pos = list_prev_entry(pos, member))
 
 /**
@@ -653,7 +662,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_for_each_entry_continue(pos, head, member) 		\
 	for (pos = list_next_entry(pos, member);			\
-	     &pos->member != (head);					\
+	     !list_entry_is_head(pos, head, member);			\
 	     pos = list_next_entry(pos, member))
 
 /**
@@ -667,7 +676,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_for_each_entry_continue_reverse(pos, head, member)		\
 	for (pos = list_prev_entry(pos, member);			\
-	     &pos->member != (head);					\
+	     !list_entry_is_head(pos, head, member);			\
 	     pos = list_prev_entry(pos, member))
 
 /**
@@ -679,7 +688,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  * Iterate over list of given type, continuing from current position.
  */
 #define list_for_each_entry_from(pos, head, member) 			\
-	for (; &pos->member != (head);					\
+	for (; !list_entry_is_head(pos, head, member);			\
 	     pos = list_next_entry(pos, member))
 
 /**
@@ -692,7 +701,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  * Iterate backwards over list of given type, continuing from current position.
  */
 #define list_for_each_entry_from_reverse(pos, head, member)		\
-	for (; &pos->member != (head);					\
+	for (; !list_entry_is_head(pos, head, member);			\
 	     pos = list_prev_entry(pos, member))
 
 /**
@@ -705,7 +714,7 @@ static inline void list_splice_tail_init(struct list_head *list,
 #define list_for_each_entry_safe(pos, n, head, member)			\
 	for (pos = list_first_entry(head, typeof(*pos), member),	\
 		n = list_next_entry(pos, member);			\
-	     &pos->member != (head); 					\
+	     !list_entry_is_head(pos, head, member); 			\
 	     pos = n, n = list_next_entry(n, member))
 
 /**
@@ -721,7 +730,7 @@ static inline void list_splice_tail_init(struct list_head *list,
 #define list_for_each_entry_safe_continue(pos, n, head, member) 		\
 	for (pos = list_next_entry(pos, member), 				\
 		n = list_next_entry(pos, member);				\
-	     &pos->member != (head);						\
+	     !list_entry_is_head(pos, head, member);				\
 	     pos = n, n = list_next_entry(n, member))
 
 /**
@@ -736,7 +745,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_for_each_entry_safe_from(pos, n, head, member) 			\
 	for (n = list_next_entry(pos, member);					\
-	     &pos->member != (head);						\
+	     !list_entry_is_head(pos, head, member);				\
 	     pos = n, n = list_next_entry(n, member))
 
 /**
@@ -752,7 +761,7 @@ static inline void list_splice_tail_init(struct list_head *list,
 #define list_for_each_entry_safe_reverse(pos, n, head, member)		\
 	for (pos = list_last_entry(head, typeof(*pos), member),		\
 		n = list_prev_entry(pos, member);			\
-	     &pos->member != (head); 					\
+	     !list_entry_is_head(pos, head, member); 			\
 	     pos = n, n = list_prev_entry(n, member))
 
 /**
-- 
2.28.0

Re: [PATCH v2] list: Add a macro to test if entry is pointing to the head

[Rojewski, Cezary]

On 2020-09-29 3:43 PM, Andy Shevchenko wrote:
> Add a macro to test if entry is pointing to the head of the list
> which is useful in cases like:
> 
>    list_for_each_entry(pos, &head, member) {
>      if (cond)
>        break;
>    }
>    if (list_entry_is_head(pos, &head, member))
>      return -ERRNO;
> 
> that allows to avoid additional variable to be added to track if loop
> has not been stopped in the middle.
> 
> While here, convert list_for_each_entry*() family of macros to use a new one.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
> v2: converted users inside list.h, dropped ambiguous description

Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>

This is a good addition to the list of helper macros found in list.h.

When looking at below:

>   /**
>    * list_for_each_entry	-	iterate over list of given type
>    * @pos:	the type * to use as a loop cursor.
> @@ -617,7 +626,7 @@ static inline void list_splice_tail_init(struct list_head *list,
>    */
>   #define list_for_each_entry(pos, head, member)				\
>   	for (pos = list_first_entry(head, typeof(*pos), member);	\
> -	     &pos->member != (head);					\
> +	     !list_entry_is_head(pos, head, member);			\
>   	     pos = list_next_entry(pos, member))
>   

it seems such helper should have been here a long time ago (notice the
usage of helpers for initial assignment and cursor update while the
loop-exit check was devoid of such).

Czarek

Re: [PATCH v2] list: Add a macro to test if entry is pointing to the head

[kernel test robot]

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CC: kbuild-all(a)lists.01.org
In-Reply-To: <20200929134342.51489-1-andriy.shevchenko@linux.intel.com>
References: <20200929134342.51489-1-andriy.shevchenko@linux.intel.com>
TO: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
TO: Cezary Rojewski <cezary.rojewski@intel.com>
TO: linux-kernel(a)vger.kernel.org
TO: Andrew Morton <akpm@linux-foundation.org>
CC: Linux Memory Management List <linux-mm@kvack.org>
CC: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Hi Andy,

I love your patch! Perhaps something to improve:

[auto build test WARNING on linux/master]
[also build test WARNING on hnaz-linux-mm/master linus/master v5.9-rc7 next-20200929]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Andy-Shevchenko/list-Add-a-macro-to-test-if-entry-is-pointing-to-the-head/20200929-214516
base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git bcf876870b95592b52519ed4aafcf9d95999bc9c
:::::: branch date: 5 hours ago
:::::: commit date: 5 hours ago
config: m68k-randconfig-s031-20200929 (attached as .config)
compiler: m68k-linux-gcc (GCC) 9.3.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # apt-get install sparse
        # sparse version: v0.6.2-201-g24bdaac6-dirty
        # https://github.com/0day-ci/linux/commit/14b9d0b5de0db1b59e77c8589c5c5701d31c7ae4
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Andy-Shevchenko/list-Add-a-macro-to-test-if-entry-is-pointing-to-the-head/20200929-214516
        git checkout 14b9d0b5de0db1b59e77c8589c5c5701d31c7ae4
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' ARCH=m68k 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

	echo
	echo "sparse warnings: (new ones prefixed by >>)"
	echo
   fs/orangefs/devorangefs-req.c: note: in included file (through include/linux/mmzone.h, include/linux/gfp.h, include/linux/slab.h, ...):
>> include/linux/spinlock.h:393:9: sparse: sparse: context imbalance in 'orangefs_devreq_read' - different lock contexts for basic block

vim +/orangefs_devreq_read +393 include/linux/spinlock.h

c2f21ce2e31286 Thomas Gleixner 2009-12-02  390  
3490565b633c70 Denys Vlasenko  2015-07-13  391  static __always_inline void spin_unlock(spinlock_t *lock)
c2f21ce2e31286 Thomas Gleixner 2009-12-02  392  {
c2f21ce2e31286 Thomas Gleixner 2009-12-02 @393  	raw_spin_unlock(&lock->rlock);
c2f21ce2e31286 Thomas Gleixner 2009-12-02  394  }
c2f21ce2e31286 Thomas Gleixner 2009-12-02  395  

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org

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