The three hash_for_each_xxx() helper iterate the hash table with help
of hlist_for_each_entry_xxx(), which breaks the loop only when obj is
NULL.
This means the check during each iteration is redundant. This patch
removes it.
Signed-off-by: Wei Yang <[email protected]>
---
include/linux/hashtable.h | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)
diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
index f6c666730b8c..a15719ed303f 100644
--- a/include/linux/hashtable.h
+++ b/include/linux/hashtable.h
@@ -124,8 +124,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @member: the name of the hlist_node within the struct
*/
#define hash_for_each(name, bkt, obj, member) \
- for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
- (bkt)++)\
+ for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
hlist_for_each_entry(obj, &name[bkt], member)
/**
@@ -136,8 +135,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @member: the name of the hlist_node within the struct
*/
#define hash_for_each_rcu(name, bkt, obj, member) \
- for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
- (bkt)++)\
+ for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
hlist_for_each_entry_rcu(obj, &name[bkt], member)
/**
@@ -150,8 +148,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @member: the name of the hlist_node within the struct
*/
#define hash_for_each_safe(name, bkt, tmp, obj, member) \
- for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
- (bkt)++)\
+ for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
hlist_for_each_entry_safe(obj, tmp, &name[bkt], member)
/**
--
2.23.0
On Wed, Oct 06, 2021 at 03:21:00PM +0000, Wei Yang wrote:
> The three hash_for_each_xxx() helper iterate the hash table with help
> of hlist_for_each_entry_xxx(), which breaks the loop only when obj is
> NULL.
>
> This means the check during each iteration is redundant. This patch
> removes it.
Are you sure that the compiler didn't already remove it? Is the code
output the same or different with this change?
How did you test this?
thanks,
greg k-h
On Thu, 07 Oct 2021, Wei Yang wrote:
> The three hash_for_each_xxx() helper iterate the hash table with help
> of hlist_for_each_entry_xxx(), which breaks the loop only when obj is
> NULL.
>
> This means the check during each iteration is redundant. This patch
> removes it.
>
> Signed-off-by: Wei Yang <[email protected]>
> ---
> include/linux/hashtable.h | 9 +++------
> 1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
> index f6c666730b8c..a15719ed303f 100644
> --- a/include/linux/hashtable.h
> +++ b/include/linux/hashtable.h
> @@ -124,8 +124,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @member: the name of the hlist_node within the struct
> */
> #define hash_for_each(name, bkt, obj, member) \
> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
> - (bkt)++)\
> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
> hlist_for_each_entry(obj, &name[bkt], member)
I think you are missing an important property of this code.
What we have here is a new loop command (hash_for_each()) that is
constructed from 2 nested loops. This sort of construct is in general
difficult to use because in C it is common to use "break" to exit a loop
early. 'break' cannot exit two levels of loop though. So if you aren't
careful, doing something like
hash_for_each() {
do something
if (some test)
break;
}
might not do what you expect. The 'break' will exit the inner loop, but
not the outer loop. That could easily lead to buggy code.
But this macro *is* careful. If the loop body *does* use break, then
the inner loop will abort but 'obj' will still be non-NULL. The test
for NULL in the outer loop causes the outer loop to abort too - as the
programmer probably expected.
So by removing the 'obj == NULL' test, you would cause any usage which
breaks out of the loop to now be incorrect.
I recommend that instead of this patch, you provide a patch which
improves the documentation to make this clear. e.g.
Note: it is safe to 'break' out of this loop even though it is a two
nested loops. The 'obj == NULL' test ensures that when the inner loop
is broken, the outer loop will break too.
Thanks,
NeilBrown
>
> /**
> @@ -136,8 +135,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @member: the name of the hlist_node within the struct
> */
> #define hash_for_each_rcu(name, bkt, obj, member) \
> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
> - (bkt)++)\
> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
> hlist_for_each_entry_rcu(obj, &name[bkt], member)
>
> /**
> @@ -150,8 +148,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @member: the name of the hlist_node within the struct
> */
> #define hash_for_each_safe(name, bkt, tmp, obj, member) \
> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
> - (bkt)++)\
> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
> hlist_for_each_entry_safe(obj, tmp, &name[bkt], member)
>
> /**
> --
> 2.23.0
>
>
On Thu, Oct 07, 2021 at 08:16:11AM +1100, NeilBrown wrote:
>On Thu, 07 Oct 2021, Wei Yang wrote:
>> The three hash_for_each_xxx() helper iterate the hash table with help
>> of hlist_for_each_entry_xxx(), which breaks the loop only when obj is
>> NULL.
>>
>> This means the check during each iteration is redundant. This patch
>> removes it.
>>
>> Signed-off-by: Wei Yang <[email protected]>
>> ---
>> include/linux/hashtable.h | 9 +++------
>> 1 file changed, 3 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
>> index f6c666730b8c..a15719ed303f 100644
>> --- a/include/linux/hashtable.h
>> +++ b/include/linux/hashtable.h
>> @@ -124,8 +124,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
>> * @member: the name of the hlist_node within the struct
>> */
>> #define hash_for_each(name, bkt, obj, member) \
>> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
>> - (bkt)++)\
>> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
>> hlist_for_each_entry(obj, &name[bkt], member)
>
>I think you are missing an important property of this code.
>What we have here is a new loop command (hash_for_each()) that is
>constructed from 2 nested loops. This sort of construct is in general
>difficult to use because in C it is common to use "break" to exit a loop
>early. 'break' cannot exit two levels of loop though. So if you aren't
>careful, doing something like
>
> hash_for_each() {
> do something
> if (some test)
> break;
> }
>
>might not do what you expect. The 'break' will exit the inner loop, but
>not the outer loop. That could easily lead to buggy code.
>
>But this macro *is* careful. If the loop body *does* use break, then
>the inner loop will abort but 'obj' will still be non-NULL. The test
>for NULL in the outer loop causes the outer loop to abort too - as the
>programmer probably expected.
>
Thanks for pointing out. I missed this case.
>So by removing the 'obj == NULL' test, you would cause any usage which
>breaks out of the loop to now be incorrect.
>
>I recommend that instead of this patch, you provide a patch which
>improves the documentation to make this clear. e.g.
>
> Note: it is safe to 'break' out of this loop even though it is a two
> nested loops. The 'obj == NULL' test ensures that when the inner loop
> is broken, the outer loop will break too.
>
Here is a draft patch based on you comment:
diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
index f6c666730b8c..2ff4cb5e6a22 100644
--- a/include/linux/hashtable.h
+++ b/include/linux/hashtable.h
@@ -116,6 +116,13 @@ static inline void hash_del_rcu(struct hlist_node *node)
hlist_del_init_rcu(node);
}
+/**
+ * Note: the following three hash_for_each[_xxx] helpers introduce a new loop
+ * command that is constructed from 2 nested loops. It is safe to 'break' out
+ * of this loop even though it is a two nested loops. The 'obj == NULL' test
+ * ensures that when the inner loop is broken, the outer loop will break too.
+ */
+
/**
* hash_for_each - iterate over a hashtable
* @name: hashtable to iterate
If you feel good, I would like to add
Sugguested-by: NeilBrown <[email protected]>
>Thanks,
>NeilBrown
>
>
>>
>> /**
>> @@ -136,8 +135,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
>> * @member: the name of the hlist_node within the struct
>> */
>> #define hash_for_each_rcu(name, bkt, obj, member) \
>> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
>> - (bkt)++)\
>> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
>> hlist_for_each_entry_rcu(obj, &name[bkt], member)
>>
>> /**
>> @@ -150,8 +148,7 @@ static inline void hash_del_rcu(struct hlist_node *node)
>> * @member: the name of the hlist_node within the struct
>> */
>> #define hash_for_each_safe(name, bkt, tmp, obj, member) \
>> - for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
>> - (bkt)++)\
>> + for ((bkt) = 0, obj = NULL; (bkt) < HASH_SIZE(name); (bkt)++) \
>> hlist_for_each_entry_safe(obj, tmp, &name[bkt], member)
>>
>> /**
>> --
>> 2.23.0
>>
>>
--
Wei Yang
Help you, Help me
On Thu, 07 Oct 2021, Wei Yang wrote:
>
> Here is a draft patch based on you comment:
>
> diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
> index f6c666730b8c..2ff4cb5e6a22 100644
> --- a/include/linux/hashtable.h
> +++ b/include/linux/hashtable.h
> @@ -116,6 +116,13 @@ static inline void hash_del_rcu(struct hlist_node *node)
> hlist_del_init_rcu(node);
> }
>
> +/**
> + * Note: the following three hash_for_each[_xxx] helpers introduce a new loop
> + * command that is constructed from 2 nested loops. It is safe to 'break' out
> + * of this loop even though it is a two nested loops. The 'obj == NULL' test
> + * ensures that when the inner loop is broken, the outer loop will break too.
> + */
> +
> /**
> * hash_for_each - iterate over a hashtable
> * @name: hashtable to iterate
>
>
> If you feel good, I would like to add
>
> Sugguested-by: NeilBrown <[email protected]>
That's definitely an improvement.
I'd probably put it in the kernel-doc comment for hash_for_each,
then in the other two just put the "it is safe" bit. Something like
the following. But I don't feel strongly about it.
I'm happy to say
Reviewed-by: NeilBrown <[email protected]>
for your patch.
Thanks,
NeilBrown
diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
index f6c666730b8c..61db940c9501 100644
--- a/include/linux/hashtable.h
+++ b/include/linux/hashtable.h
@@ -122,6 +122,10 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @bkt: integer to use as bucket loop cursor
* @obj: the type * to use as a loop cursor for each entry
* @member: the name of the hlist_node within the struct
+ *
+ * Note: It is safe to 'break' out of this loop even though it is a two
+ * nested loops. The 'obj == NULL' test ensures that when the inner loop
+ * is broken, the outer loop will break too.
*/
#define hash_for_each(name, bkt, obj, member) \
for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
@@ -134,6 +138,8 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @bkt: integer to use as bucket loop cursor
* @obj: the type * to use as a loop cursor for each entry
* @member: the name of the hlist_node within the struct
+ *
+ * It is safe to 'break' out of this loop.
*/
#define hash_for_each_rcu(name, bkt, obj, member) \
for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
@@ -148,6 +154,8 @@ static inline void hash_del_rcu(struct hlist_node *node)
* @tmp: a &struct hlist_node used for temporary storage
* @obj: the type * to use as a loop cursor for each entry
* @member: the name of the hlist_node within the struct
+ *
+ * It is safe to 'break' out of this loop.
*/
#define hash_for_each_safe(name, bkt, tmp, obj, member) \
for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
On Thu, Oct 07, 2021 at 11:50:22AM +1100, NeilBrown wrote:
>On Thu, 07 Oct 2021, Wei Yang wrote:
>>
>> Here is a draft patch based on you comment:
>>
>> diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
>> index f6c666730b8c..2ff4cb5e6a22 100644
>> --- a/include/linux/hashtable.h
>> +++ b/include/linux/hashtable.h
>> @@ -116,6 +116,13 @@ static inline void hash_del_rcu(struct hlist_node *node)
>> hlist_del_init_rcu(node);
>> }
>>
>> +/**
>> + * Note: the following three hash_for_each[_xxx] helpers introduce a new loop
>> + * command that is constructed from 2 nested loops. It is safe to 'break' out
>> + * of this loop even though it is a two nested loops. The 'obj == NULL' test
>> + * ensures that when the inner loop is broken, the outer loop will break too.
>> + */
>> +
>> /**
>> * hash_for_each - iterate over a hashtable
>> * @name: hashtable to iterate
>>
>>
>> If you feel good, I would like to add
>>
>> Sugguested-by: NeilBrown <[email protected]>
>
>That's definitely an improvement.
>
>I'd probably put it in the kernel-doc comment for hash_for_each,
>then in the other two just put the "it is safe" bit. Something like
>the following. But I don't feel strongly about it.
>I'm happy to say
> Reviewed-by: NeilBrown <[email protected]>
>
Thanks for your detailed instruction :-)
>for your patch.
>
>Thanks,
>NeilBrown
>
>
>diff --git a/include/linux/hashtable.h b/include/linux/hashtable.h
>index f6c666730b8c..61db940c9501 100644
>--- a/include/linux/hashtable.h
>+++ b/include/linux/hashtable.h
>@@ -122,6 +122,10 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @bkt: integer to use as bucket loop cursor
> * @obj: the type * to use as a loop cursor for each entry
> * @member: the name of the hlist_node within the struct
>+ *
>+ * Note: It is safe to 'break' out of this loop even though it is a two
>+ * nested loops. The 'obj == NULL' test ensures that when the inner loop
>+ * is broken, the outer loop will break too.
> */
> #define hash_for_each(name, bkt, obj, member) \
> for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
>@@ -134,6 +138,8 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @bkt: integer to use as bucket loop cursor
> * @obj: the type * to use as a loop cursor for each entry
> * @member: the name of the hlist_node within the struct
>+ *
>+ * It is safe to 'break' out of this loop.
> */
> #define hash_for_each_rcu(name, bkt, obj, member) \
> for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
>@@ -148,6 +154,8 @@ static inline void hash_del_rcu(struct hlist_node *node)
> * @tmp: a &struct hlist_node used for temporary storage
> * @obj: the type * to use as a loop cursor for each entry
> * @member: the name of the hlist_node within the struct
>+ *
>+ * It is safe to 'break' out of this loop.
> */
> #define hash_for_each_safe(name, bkt, tmp, obj, member) \
> for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
--
Wei Yang
Help you, Help me