linux-kernel - Re: [PATCH 2/2] Update sem

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Date:	Thu, 26 Sep 2013 16:53:18 +0800
From:	Jia He <jiakernel@...il.com>
To:	Manfred Spraul <manfred@...orfullife.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 2/2] Update sem_otime for all operations

Tested-by: Jia He<jiakernel@...il.com>

# cat /proc/sysvipc/sem 
       key      semid perms      nsems   uid   gid  cuid  cgid      otime      ctime
        -1      32768   666          1     0     0     0     0 1380185570 1380185570


On Thu, 26 Sep 2013 07:08:55 +0200 from manfred@...orfullife.com wrote:
> Hi Jia,
>
> Could you check if the patch below resolves the bug you have reported?
> Just this patch, i.e. without your proposal.
>
> I want to leave the optimization for the get_seconds() call:
> We must update sem_otime in two places anyway
> (either perform_atomic_semop() and exit_sem() or
> do_smart_update() and semtimedop())
>
> --
> 	Manfred
>
> In commit 0a2b9d4c,the update of semaphore's sem_otime(last semop time)
> was moved to one central position (do_smart_update).
>
> But: Since do_smart_update() is only called for operations that modify
> the array, this means that wait-for-zero semops do not update sem_otime
> anymore.
>
> The fix is simple:
> Non-alter operations must update sem_otime.
>
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
> Reported-by: Jia He <jiakernel@...il.com>
> ---
>  ipc/sem.c | 41 +++++++++++++++++++++++++++++------------
>  1 file changed, 29 insertions(+), 12 deletions(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index e5d9bb8..ec83f79 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -910,6 +910,24 @@ again:
>  }
>  
>  /**
> + * set_semotime(sma, sops) - set sem_otime
> + * @sma: semaphore array
> + * @sops: operations that modified the array, may be NULL
> + *
> + * sem_otime is replicated to avoid cache line trashing.
> + * This function sets one instance to the current time.
> + */
> +static void set_semotime(struct sem_array *sma, struct sembuf *sops)
> +{
> +	if (sops == NULL) {
> +		sma->sem_base[0].sem_otime = get_seconds();
> +	} else {
> +		sma->sem_base[sops[0].sem_num].sem_otime =
> +							get_seconds();
> +	}
> +}
> +
> +/**
>   * do_smart_update(sma, sops, nsops, otime, pt) - optimized update_queue
>   * @sma: semaphore array
>   * @sops: operations that were performed
> @@ -959,17 +977,10 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
>  			}
>  		}
>  	}
> -	if (otime) {
> -		if (sops == NULL) {
> -			sma->sem_base[0].sem_otime = get_seconds();
> -		} else {
> -			sma->sem_base[sops[0].sem_num].sem_otime =
> -								get_seconds();
> -		}
> -	}
> +	if (otime)
> +		set_semotime(sma, sops);
>  }
>  
> -
>  /* The following counts are associated to each semaphore:
>   *   semncnt        number of tasks waiting on semval being nonzero
>   *   semzcnt        number of tasks waiting on semval being zero
> @@ -1831,10 +1842,16 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>  
>  	error = perform_atomic_semop(sma, sops, nsops, un,
>  					task_tgid_vnr(current));
> -	if (error <= 0) {
> -		if (alter && error == 0)
> +	if (error == 0) {
> +		/* If the operation was successful, then do
> +		 * the required updates.
> +		 */
> +		if (alter)
>  			do_smart_update(sma, sops, nsops, 1, &tasks);
> -
> +		else
> +			set_semotime(sma, sops);
> +	}
> +	if (error <= 0) {
>  		goto out_unlock_free;
>  	}
>  

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