linux-kernel - Re: [PATCH 1/5] ipc/sem: do not call wake_sem_queue

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Date:   Tue, 13 Sep 2016 06:17:43 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Davidlohr Bueso <dave@...olabs.net>, akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/5] ipc/sem: do not call wake_sem_queue_do() prematurely

Hi Davidlohr,

On 09/12/2016 01:53 PM, Davidlohr Bueso wrote:
> ... as this call should obviously be paired with its _prepare()
> counterpart. At least whenever possible, as there is no harm in
> calling it bogusly as we do now in a few places.
I would define the interface differently:
WAKE_Q creates an initialized wake queue. There is no need to track if 
any tasks were added to the wake queue, it is safe to call wake_up_q().
So especially for error paths, there is no need to optimize out calls to 
wake_up_q()
>   Immediate error
> semop(2) paths that are far from ever having the task block can
> be simplified and avoid a few unnecessary loads on their way out
> of the call as it is not deeply nested.
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
>   ipc/sem.c | 19 ++++++++++++-------
>   1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 5e318c5f749d..a4e8bb2fae38 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1887,16 +1887,22 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>   	}
>   
>   	error = -EFBIG;
> -	if (max >= sma->sem_nsems)
> -		goto out_rcu_wakeup;
> +	if (max >= sma->sem_nsems) {
> +		rcu_read_unlock();
> +		goto out_free;
> +	}
>   
>   	error = -EACCES;
> -	if (ipcperms(ns, &sma->sem_perm, alter ? S_IWUGO : S_IRUGO))
> -		goto out_rcu_wakeup;
> +	if (ipcperms(ns, &sma->sem_perm, alter ? S_IWUGO : S_IRUGO)) {
> +		rcu_read_unlock();
> +		goto out_free;
> +	}
>   
Is this really better/simpler?
You replace "if (error) goto cleanup" with "if (error) {cleanup_1(); 
goto cleanup_2()}".

 From my point of view, this just increases the risks that some cleanup 
steps are forgotten.

--
     Manfred