[PATCH] pid: remove redundant condition

[PATCH] pid: remove redundant condition

[Liu Ping Fan]

From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
 kernel/pid.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 9f08dfa..e4ca244 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -248,8 +248,7 @@ void put_pid(struct pid *pid)
 		return;
 
 	ns = pid->numbers[pid->level].ns;
-	if ((atomic_read(&pid->count) == 1) ||
-	     atomic_dec_and_test(&pid->count)) {
+	if (atomic_dec_and_test(&pid->count) {
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
 	}
-- 
1.7.4.4

Re: [PATCH] pid: remove redundant condition

[Linus Torvalds]

On Thu, Apr 5, 2012 at 11:16 PM, Liu Ping Fan <kernelfans@gmail.com> wrote:
> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>
> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> ---
>  kernel/pid.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 9f08dfa..e4ca244 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -248,8 +248,7 @@ void put_pid(struct pid *pid)
>                return;
>
>        ns = pid->numbers[pid->level].ns;
> -       if ((atomic_read(&pid->count) == 1) ||
> -            atomic_dec_and_test(&pid->count)) {
> +       if (atomic_dec_and_test(&pid->count) {
>                kmem_cache_free(ns->pid_cachep, pid);
>                put_pid_ns(ns);
>        }

No, this isn't necessarily redundant.

The atomic_dec_and_test() instruction can be very expensive. So if
count was 1 before, we're the last user, and we can free it without
the expensive atomic_dec_and_test().

However, if we are *not* the last user, then we can race with another
user decrementing the count, so now we need to use the expensive
version.

Now, whether that special case is really worth it or not, I don't
know. But it's not redundant.

                    Linus

Re: [PATCH] pid: remove redundant condition

[Liu ping fan]

On Fri, Apr 6, 2012 at 2:26 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Apr 5, 2012 at 11:16 PM, Liu Ping Fan <kernelfans@gmail.com> wrote:
>> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>>
>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>> ---
>>  kernel/pid.c |    3 +--
>>  1 files changed, 1 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/pid.c b/kernel/pid.c
>> index 9f08dfa..e4ca244 100644
>> --- a/kernel/pid.c
>> +++ b/kernel/pid.c
>> @@ -248,8 +248,7 @@ void put_pid(struct pid *pid)
>>                return;
>>
>>        ns = pid->numbers[pid->level].ns;
>> -       if ((atomic_read(&pid->count) == 1) ||
>> -            atomic_dec_and_test(&pid->count)) {
>> +       if (atomic_dec_and_test(&pid->count) {
>>                kmem_cache_free(ns->pid_cachep, pid);
>>                put_pid_ns(ns);
>>        }
>
> No, this isn't necessarily redundant.
>
> The atomic_dec_and_test() instruction can be very expensive. So if
Oh, got it, thanks.

Regards,
pingfan
> count was 1 before, we're the last user, and we can free it without
> the expensive atomic_dec_and_test().
>
> However, if we are *not* the last user, then we can race with another
> user decrementing the count, so now we need to use the expensive
> version.
>
> Now, whether that special case is really worth it or not, I don't
> know. But it's not redundant.
>
>                    Linus