[PATCH] RDS: Simplify code

[PATCH] RDS: Simplify code

[Christophe JAILLET]

Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
'list_splice_init'.

This has been spotted with the following coccinelle script:
/////
@@
expression y,z;
@@

-   list_splice(y,z);
-   INIT_LIST_HEAD(y);
+   list_splice_init(y,z);

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
 net/rds/loop.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/rds/loop.c b/net/rds/loop.c
index f2bf78de5688..c3e6da4fdf97 100644
--- a/net/rds/loop.c
+++ b/net/rds/loop.c
@@ -167,8 +167,7 @@ void rds_loop_exit(void)
 
 	/* avoid calling conn_destroy with irqs off */
 	spin_lock_irq(&loop_conns_lock);
-	list_splice(&loop_conns, &tmp_list);
-	INIT_LIST_HEAD(&loop_conns);
+	list_splice_init(&loop_conns, &tmp_list);
 	spin_unlock_irq(&loop_conns_lock);
 
 	list_for_each_entry_safe(lc, _lc, &tmp_list, loop_node) {
-- 
2.7.4

Re: [PATCH] RDS: Simplify code

[Leon Romanovsky]

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On Sat, Sep 03, 2016 at 07:33:29AM +0200, Christophe JAILLET wrote:
> Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
> 'list_splice_init'.

It is not 100% accurate

list_splice(y, z)
INIT_LIST_HEAD(y)

==>

if (!list_empty(y))
     __list_splice(y, z, z>next);
INIT_LIST_HEAD(y)

and not

if (!list_empty(y)) {
     __list_splice(y, z, z>next);
     INIT_LIST_HEAD(y)
}

as list_splice_init will do.


>
> This has been spotted with the following coccinelle script:
> /////
> @@
> expression y,z;
> @@
>
> -   list_splice(y,z);
> -   INIT_LIST_HEAD(y);
> +   list_splice_init(y,z);
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
>  net/rds/loop.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/net/rds/loop.c b/net/rds/loop.c
> index f2bf78de5688..c3e6da4fdf97 100644
> --- a/net/rds/loop.c
> +++ b/net/rds/loop.c
> @@ -167,8 +167,7 @@ void rds_loop_exit(void)
>
>  	/* avoid calling conn_destroy with irqs off */
>  	spin_lock_irq(&loop_conns_lock);
> -	list_splice(&loop_conns, &tmp_list);
> -	INIT_LIST_HEAD(&loop_conns);
> +	list_splice_init(&loop_conns, &tmp_list);
>  	spin_unlock_irq(&loop_conns_lock);
>
>  	list_for_each_entry_safe(lc, _lc, &tmp_list, loop_node) {
> --
> 2.7.4
>

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Re: [PATCH] RDS: Simplify code

[Christophe JAILLET]

Le 04/09/2016 à 14:20, Leon Romanovsky a écrit :
> On Sat, Sep 03, 2016 at 07:33:29AM +0200, Christophe JAILLET wrote:
>> Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
>> 'list_splice_init'.
> It is not 100% accurate
>
> list_splice(y, z)
> INIT_LIST_HEAD(y)
>
> ==>
>
> if (!list_empty(y))
>       __list_splice(y, z, z>next);
> INIT_LIST_HEAD(y)
>
> and not
>
> if (!list_empty(y)) {
>       __list_splice(y, z, z>next);
>       INIT_LIST_HEAD(y)
> }
>
> as list_splice_init will do.
>
You are right but if you dig further you will see that calling 
INIT_LIST_HEAD on an empty list is a no-op (AFAIK).
And if this list was not already correctly initialized, then you would 
have some other troubles.

CJ

Re: [PATCH] RDS: Simplify code

[Leon Romanovsky]

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On Sun, Sep 04, 2016 at 05:57:20PM +0200, Christophe JAILLET wrote:
> Le 04/09/2016 à 14:20, Leon Romanovsky a écrit :
> >On Sat, Sep 03, 2016 at 07:33:29AM +0200, Christophe JAILLET wrote:
> >>Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
> >>'list_splice_init'.
> >It is not 100% accurate
> >
> >list_splice(y, z)
> >INIT_LIST_HEAD(y)
> >
> >==>
> >
> >if (!list_empty(y))
> >      __list_splice(y, z, z>next);
> >INIT_LIST_HEAD(y)
> >
> >and not
> >
> >if (!list_empty(y)) {
> >      __list_splice(y, z, z>next);
> >      INIT_LIST_HEAD(y)
> >}
> >
> >as list_splice_init will do.
> >
> You are right but if you dig further you will see that calling
> INIT_LIST_HEAD on an empty list is a no-op (AFAIK).
> And if this list was not already correctly initialized, then you would have
> some other troubles.

Thank you for the suggestion,
It looks like the code after that can be skipped in case of loop_conns
list is empty, the tmp_list will be empty too.

174         list_for_each_entry_safe(lc, _lc, &tmp_list, loop_node) {
175                 WARN_ON(lc->conn->c_passive);
176                 rds_conn_destroy(lc->conn);
177         }

>
> CJ
>

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Re: [PATCH] RDS: Simplify code

[Leon Romanovsky]

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On Mon, Sep 05, 2016 at 06:38:21AM +0200, Christophe JAILLET wrote:
> Le 04/09/2016 à 20:23, Leon Romanovsky a écrit :
> >On Sun, Sep 04, 2016 at 05:57:20PM +0200, Christophe JAILLET wrote:
> >>Le 04/09/2016 à 14:20, Leon Romanovsky a écrit :
> >>>On Sat, Sep 03, 2016 at 07:33:29AM +0200, Christophe JAILLET wrote:
> >>>>Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
> >>>>'list_splice_init'.
> >>>It is not 100% accurate
> >>>
> >>>list_splice(y, z)
> >>>INIT_LIST_HEAD(y)
> >>>
> >>>==>
> >>>
> >>>if (!list_empty(y))
> >>>      __list_splice(y, z, z>next);
> >>>INIT_LIST_HEAD(y)
> >>>
> >>>and not
> >>>
> >>>if (!list_empty(y)) {
> >>>      __list_splice(y, z, z>next);
> >>>      INIT_LIST_HEAD(y)
> >>>}
> >>>
> >>>as list_splice_init will do.
> >>>
> >>You are right but if you dig further you will see that calling
> >>INIT_LIST_HEAD on an empty list is a no-op (AFAIK).
> >>And if this list was not already correctly initialized, then you would have
> >>some other troubles.
> >Thank you for the suggestion,
> >It looks like the code after that can be skipped in case of loop_conns
> >list is empty, the tmp_list will be empty too.
> >
> >174         list_for_each_entry_safe(lc, _lc, &tmp_list, loop_node) {
> >175                 WARN_ON(lc->conn->c_passive);
> >176                 rds_conn_destroy(lc->conn);
> >177         }
> Yes, but this would require some more code and test. This function doesn't
> seem to be in a hot path, so I'm not sure that the added complexity would
> worth it.
> It would require a new 'list_empty()' test and some code rearrangement.
>
> I suppose that testing for emptiness at the beginning or going through a
> list_for_each_entry_safe on a empty list (which will exit immediately and do
> nothing) is more or less the same in term of speed. So keep the code simple
> and readable.

I would expect one list_empty check at the beginning and return
immediately, but anyway it doesn't matter.

>
> CJ
>
>
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Re: [PATCH] RDS: Simplify code

[santosh.shilimkar]

On 9/4/16 11:23 AM, Leon Romanovsky wrote:
> On Sun, Sep 04, 2016 at 05:57:20PM +0200, Christophe JAILLET wrote:
>> Le 04/09/2016 à 14:20, Leon Romanovsky a écrit :
>>> On Sat, Sep 03, 2016 at 07:33:29AM +0200, Christophe JAILLET wrote:
>>>> Calling 'list_splice' followed by 'INIT_LIST_HEAD' is equivalent to
>>>> 'list_splice_init'.
>>> It is not 100% accurate
>>>
>>> list_splice(y, z)
>>> INIT_LIST_HEAD(y)
>>>
>>> ==>
>>>
>>> if (!list_empty(y))
>>>      __list_splice(y, z, z>next);
>>> INIT_LIST_HEAD(y)
>>>
>>> and not
>>>
>>> if (!list_empty(y)) {
>>>      __list_splice(y, z, z>next);
>>>      INIT_LIST_HEAD(y)
>>> }
>>>
>>> as list_splice_init will do.
>>>
>> You are right but if you dig further you will see that calling
>> INIT_LIST_HEAD on an empty list is a no-op (AFAIK).
>> And if this list was not already correctly initialized, then you would have
>> some other troubles.
>
> Thank you for the suggestion,
> It looks like the code after that can be skipped in case of loop_conns
> list is empty, the tmp_list will be empty too.
>
> 174         list_for_each_entry_safe(lc, _lc, &tmp_list, loop_node) {
> 175                 WARN_ON(lc->conn->c_passive);
> 176                 rds_conn_destroy(lc->conn);
> 177         }
>

Thanks for trying. As already pointed, your change doesn't simplify
much rather change the behavior. The loop cursor already takes care
of list empty case. I don't see any reason to change that code.

Regards,
Santosh