Re: [PATCH 3/4] ipc/sem.c: convert sem_array.sem_pending to struct list_head

[Nadia Derbey <Nadia.Derbey@bull.net>]

Manfred Spraul wrote:
> sem_array.sem_pending is a double linked list, the attached
> patch converts it to struct list_head.
> 
> Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>

Reviewed-By: Nadia Derbey <Nadia.Derbey@bull.net>

2 small comments embedded.

> ---
>  include/linux/sem.h |   12 +++----
>  ipc/sem.c           |   90 ++++++++++++++++++--------------------------------
>  2 files changed, 38 insertions(+), 64 deletions(-)
> 
> diff --git a/include/linux/sem.h b/include/linux/sem.h
> index 87756ef..d425993 100644
> --- a/include/linux/sem.h
> +++ b/include/linux/sem.h
> @@ -93,21 +93,19 @@ struct sem_array {
>  	time_t			sem_otime;	/* last semop time */
>  	time_t			sem_ctime;	/* last change time */
>  	struct sem		*sem_base;	/* ptr to first semaphore in array */
> -	struct sem_queue	*sem_pending;	/* pending operations to be processed */
> -	struct sem_queue	**sem_pending_last; /* last pending operation */
> +	struct list_head	sem_pending;	/* pending operations to be processed */
>  	struct list_head	list_id;	/* undo requests on this array */
>  	unsigned long		sem_nsems;	/* no. of semaphores in array */
>  };
>  
>  /* One queue for each sleeping process in the system. */
>  struct sem_queue {
> -	struct sem_queue *	next;	 /* next entry in the queue */
> -	struct sem_queue **	prev;	 /* previous entry in the queue, *(q->prev) == q */
> -	struct task_struct*	sleeper; /* this process */
> -	struct sem_undo *	undo;	 /* undo structure */
> +	struct list_head	list;	 /* queue of pending operations */
> +	struct task_struct	*sleeper; /* this process */
> +	struct sem_undo		*undo;	 /* undo structure */
>  	int    			pid;	 /* process id of requesting process */
>  	int    			status;	 /* completion status of operation */
> -	struct sembuf *		sops;	 /* array of pending operations */
> +	struct sembuf		*sops;	 /* array of pending operations */
>  	int			nsops;	 /* number of operations */
>  	int			alter;   /* does the operation alter the array? */
>  };
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 8cd96f1..38996c0 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -272,8 +272,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
>  	ns->used_sems += nsems;
>  
>  	sma->sem_base = (struct sem *) &sma[1];
> -	/* sma->sem_pending = NULL; */
> -	sma->sem_pending_last = &sma->sem_pending;
> +	INIT_LIST_HEAD(&sma->sem_pending);
>  	INIT_LIST_HEAD(&sma->list_id);
>  	sma->sem_nsems = nsems;
>  	sma->sem_ctime = get_seconds();
> @@ -331,38 +330,6 @@ asmlinkage long sys_semget(key_t key, int nsems, int semflg)
>  	return ipcget(ns, &sem_ids(ns), &sem_ops, &sem_params);
>  }
>  
> -/* Manage the doubly linked list sma->sem_pending as a FIFO:
> - * insert new queue elements at the tail sma->sem_pending_last.
> - */
> -static inline void append_to_queue (struct sem_array * sma,
> -				    struct sem_queue * q)
> -{
> -	*(q->prev = sma->sem_pending_last) = q;
> -	*(sma->sem_pending_last = &q->next) = NULL;
> -}
> -
> -static inline void prepend_to_queue (struct sem_array * sma,
> -				     struct sem_queue * q)
> -{
> -	q->next = sma->sem_pending;
> -	*(q->prev = &sma->sem_pending) = q;
> -	if (q->next)
> -		q->next->prev = &q->next;
> -	else /* sma->sem_pending_last == &sma->sem_pending */
> -		sma->sem_pending_last = &q->next;
> -}
> -
> -static inline void remove_from_queue (struct sem_array * sma,
> -				      struct sem_queue * q)
> -{
> -	*(q->prev) = q->next;
> -	if (q->next)
> -		q->next->prev = q->prev;
> -	else /* sma->sem_pending_last == &q->next */
> -		sma->sem_pending_last = q->prev;
> -	q->prev = NULL; /* mark as removed */
> -}
> -
>  /*
>   * Determine whether a sequence of semaphore operations would succeed
>   * all at once. Return 0 if yes, 1 if need to sleep, else return error code.
> @@ -438,16 +405,15 @@ static void update_queue (struct sem_array * sma)
>  	int error;
>  	struct sem_queue * q;
>  
> -	q = sma->sem_pending;
> -	while(q) {
> +	q = list_entry(sma->sem_pending.next, struct sem_queue, list);
> +	while(&q->list != &sma->sem_pending) {

I guess here you are not using list_first_entry() because the pending 
requests might be empty?

>  		error = try_atomic_semop(sma, q->sops, q->nsops,
>  					 q->undo, q->pid);
>  
>  		/* Does q->sleeper still need to sleep? */
>  		if (error <= 0) {
>  			struct sem_queue *n;
> -			remove_from_queue(sma,q);
> -			q->status = IN_WAKEUP;
> +
>  			/*
>  			 * Continue scanning. The next operation
>  			 * that must be checked depends on the type of the
> @@ -458,11 +424,24 @@ static void update_queue (struct sem_array * sma)
>  			 *   for semaphore values to become 0.
>  			 * - if the operation didn't modify the array,
>  			 *   then just continue.
> +			 * The order of list_del() and reading ->next
> +			 * is crucial: In the former case, the list_del()
> +			 * must be done first [because we might be the
> +			 * first entry in ->sem_pending], in the latter
> +			 * case the list_del() must be done last
> +			 * [because the list is invalid after the list_del()]
>  			 */
> -			if (q->alter)
> -				n = sma->sem_pending;
> -			else
> -				n = q->next;
> +			if (q->alter) {
> +				list_del(&q->list);
> +				n = list_entry(sma->sem_pending.next, struct sem_queue, list);
> +			} else {
> +				n = list_entry(q->list.next, struct sem_queue, list);
> +				list_del(&q->list);
> +			}
> +
> +			/* wake up the waiting thread */
> +			q->status = IN_WAKEUP;
> +
>  			wake_up_process(q->sleeper);
>  			/* hands-off: q will disappear immediately after
>  			 * writing q->status.
> @@ -471,7 +450,7 @@ static void update_queue (struct sem_array * sma)
>  			q->status = error;
>  			q = n;
>  		} else {
> -			q = q->next;
> +			q = list_entry(q->list.next, struct sem_queue, list);
>  		}
>  	}
>  }
> @@ -491,7 +470,7 @@ static int count_semncnt (struct sem_array * sma, ushort semnum)
>  	struct sem_queue * q;
>  
>  	semncnt = 0;
> -	for (q = sma->sem_pending; q; q = q->next) {
> +	list_for_each_entry(q, &sma->sem_pending, list) {
>  		struct sembuf * sops = q->sops;
>  		int nsops = q->nsops;
>  		int i;
> @@ -503,13 +482,14 @@ static int count_semncnt (struct sem_array * sma, ushort semnum)
>  	}
>  	return semncnt;
>  }
> +
>  static int count_semzcnt (struct sem_array * sma, ushort semnum)
>  {
>  	int semzcnt;
>  	struct sem_queue * q;
>  
>  	semzcnt = 0;
> -	for (q = sma->sem_pending; q; q = q->next) {
> +	list_for_each_entry(q, &sma->sem_pending, list) {
>  		struct sembuf * sops = q->sops;
>  		int nsops = q->nsops;
>  		int i;
> @@ -529,7 +509,7 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum)
>  static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
>  {
>  	struct sem_undo *un;
> -	struct sem_queue *q;
> +	struct sem_queue *q, *t;
>  	struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm);
>  
>  	/* Invalidate the existing undo structures for this semaphore set.
> @@ -541,17 +521,14 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
>  		un->semid = -1;
>  
>  	/* Wake up all pending processes and let them fail with EIDRM. */
> -	q = sma->sem_pending;
> -	while(q) {
> -		struct sem_queue *n;
> -		/* lazy remove_from_queue: we are killing the whole queue */
> -		q->prev = NULL;
> -		n = q->next;
> +
> +	list_for_each_entry_safe(q, t, &sma->sem_pending, list) {
> +		list_del(&q->list);
> +
>  		q->status = IN_WAKEUP;
>  		wake_up_process(q->sleeper); /* doesn't sleep */
>  		smp_wmb();
>  		q->status = -EIDRM;	/* hands-off q */
> -		q = n;
>  	}
>  
>  	/* Remove the semaphore set from the IDR */
> @@ -1166,9 +1143,9 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
>  	queue.pid = task_tgid_vnr(current);
>  	queue.alter = alter;
>  	if (alter)
> -		append_to_queue(sma ,&queue);
> +		list_add_tail(&queue.list, &sma->sem_pending);
>  	else
> -		prepend_to_queue(sma ,&queue);
> +		list_add(&queue.list, &sma->sem_pending);
>  
>  	queue.status = -EINTR;
>  	queue.sleeper = current;
> @@ -1194,7 +1171,6 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
>  
>  	sma = sem_lock(ns, semid);
>  	if (IS_ERR(sma)) {
> -		BUG_ON(queue.prev != NULL);

Instead of removing it why not replacing the bug_ON() by a check on the 
queue still being linked?

>  		error = -EIDRM;
>  		goto out_free;
>  	}
> @@ -1212,7 +1188,7 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
>  	 */
>  	if (timeout && jiffies_left == 0)
>  		error = -EAGAIN;
> -	remove_from_queue(sma,&queue);
> +	list_del(&queue.list);
>  	goto out_unlock_free;
>  
>  out_unlock_free:

Will have a look at the last patch + do some tests tomorrow.

Regards,
Nadia