[Devel] Re: [PATCH 2.6.24-rc8-mm1 14/15] (RFC) IPC/semaphores: prepare semundo code to work on another task than current

[Serge E. Hallyn]

Quoting pierre.peiffer at bull.net (pierre.peiffer at bull.net):
> From: Pierre Peiffer <pierre.peiffer at bull.net>
> 
> In order to modify the semundo-list of a task from procfs, we must be able to
> work on any target task.
> But all the existing code playing with the semundo-list, currently works
> only on the 'current' task, and does not allow to specify any target task.
> 
> This patch changes all these routines to allow them to work on a specified
> task, passed in parameter, instead of current.
> 
> This is mainly a preparation for the semundo_write() operation, on the
> /proc/<pid>/semundo file, as provided in the next patch.
> 
> Signed-off-by: Pierre Peiffer <pierre.peiffer at bull.net>
> ---
> 
>  ipc/sem.c |   90 ++++++++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 68 insertions(+), 22 deletions(-)
> 
> Index: b/ipc/sem.c
> ===================================================================
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1017,8 +1017,9 @@ asmlinkage long sys_semctl (int semid, i
>  }
> 
>  /* If the task doesn't already have a undo_list, then allocate one
> - * here.  We guarantee there is only one thread using this undo list,
> - * and current is THE ONE
> + * here.
> + * The target task (tsk) is current in the general case, except when
> + * accessed from the procfs (ie when writting to /proc/<pid>/semundo)
>   *
>   * If this allocation and assignment succeeds, but later
>   * portions of this code fail, there is no need to free the sem_undo_list.
> @@ -1026,22 +1027,60 @@ asmlinkage long sys_semctl (int semid, i
>   * at exit time.
>   *
>   * This can block, so callers must hold no locks.
> + *
> + * Note: task_lock is used to synchronize 1. several possible concurrent
> + * creations and 2. the free of the undo_list (done when the task using it
> + * exits). In the second case, we check the PF_EXITING flag to not create
> + * an undo_list for a task which has exited.
> + * If there already is an undo_list for this task, there is no need
> + * to held the task-lock to retrieve it, as the pointer can not change
> + * afterwards.
>   */
> -static inline int get_undo_list(struct sem_undo_list **undo_listp)
> +static inline int get_undo_list(struct task_struct *tsk,
> +				struct sem_undo_list **ulp)
>  {
> -	struct sem_undo_list *undo_list;
> +	if (tsk->sysvsem.undo_list == NULL) {
> +		struct sem_undo_list *undo_list;

Hmm, this is weird.  If there was no undo_list and
tsk!=current, you set the refcnt to 2.  But if there was an
undo list and tsk!=current, where do you inc the refcnt?

> 
> -	undo_list = current->sysvsem.undo_list;
> -	if (!undo_list) {
> -		undo_list = kzalloc(sizeof(*undo_list), GFP_KERNEL);
> +		/* we must alloc a new one */
> +		undo_list = kmalloc(sizeof(*undo_list), GFP_KERNEL);
>  		if (undo_list == NULL)
>  			return -ENOMEM;
> +
> +		task_lock(tsk);
> +
> +		/* check again if there is an undo_list for this task */
> +		if (tsk->sysvsem.undo_list) {
> +			if (tsk != current)
> +				atomic_inc(&tsk->sysvsem.undo_list->refcnt);
> +			task_unlock(tsk);
> +			kfree(undo_list);
> +			goto out;
> +		}
> +
>  		spin_lock_init(&undo_list->lock);
> -		atomic_set(&undo_list->refcnt, 1);
> -		undo_list->ns = get_ipc_ns(current->nsproxy->ipc_ns);
> -		current->sysvsem.undo_list = undo_list;
> +		/*
> +		 * If tsk is not current (meaning that current is creating
> +		 * a semundo_list for a target task through procfs), and if
> +		 * it's not being exited then refcnt must be 2: the target
> +		 * task tsk + current.
> +		 */
> +		if (tsk == current)
> +			atomic_set(&undo_list->refcnt, 1);
> +		else if (!(tsk->flags & PF_EXITING))
> +			atomic_set(&undo_list->refcnt, 2);
> +		else {
> +			task_unlock(tsk);
> +			kfree(undo_list);
> +			return -EINVAL;
> +		}
> +		undo_list->ns = get_ipc_ns(tsk->nsproxy->ipc_ns);
> +		undo_list->proc_list = NULL;
> +		tsk->sysvsem.undo_list = undo_list;
> +		task_unlock(tsk);
>  	}
> -	*undo_listp = undo_list;
> +out:
> +	*ulp = tsk->sysvsem.undo_list;
>  	return 0;
>  }
> 
> @@ -1065,17 +1104,12 @@ static struct sem_undo *lookup_undo(stru
>  	return un;
>  }
> 
> -static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
> +static struct sem_undo *find_undo(struct sem_undo_list *ulp, int semid)
>  {
>  	struct sem_array *sma;
> -	struct sem_undo_list *ulp;
>  	struct sem_undo *un, *new;
> +	struct ipc_namespace *ns;
>  	int nsems;
> -	int error;
> -
> -	error = get_undo_list(&ulp);
> -	if (error)
> -		return ERR_PTR(error);
> 
>  	spin_lock(&ulp->lock);
>  	un = lookup_undo(ulp, semid);
> @@ -1083,6 +1117,8 @@ static struct sem_undo *find_undo(struct
>  	if (likely(un!=NULL))
>  		goto out;
> 
> +	ns = ulp->ns;
> +
>  	/* no undo structure around - allocate one. */
>  	sma = sem_lock_check(ns, semid);
>  	if (IS_ERR(sma))
> @@ -1133,6 +1169,7 @@ asmlinkage long sys_semtimedop(int semid
>  	struct sem_array *sma;
>  	struct sembuf fast_sops[SEMOPM_FAST];
>  	struct sembuf* sops = fast_sops, *sop;
> +	struct sem_undo_list *ulp;
>  	struct sem_undo *un;
>  	int undos = 0, alter = 0, max;
>  	struct sem_queue queue;
> @@ -1177,9 +1214,13 @@ asmlinkage long sys_semtimedop(int semid
>  			alter = 1;
>  	}
> 
> +	error = get_undo_list(current, &ulp);
> +	if (error)
> +		goto out_free;
> +
>  retry_undos:
>  	if (undos) {
> -		un = find_undo(ns, semid);
> +		un = find_undo(ulp, semid);
>  		if (IS_ERR(un)) {
>  			error = PTR_ERR(un);
>  			goto out_free;
> @@ -1305,7 +1346,7 @@ int copy_semundo(unsigned long clone_fla
>  	int error;
> 
>  	if (clone_flags & CLONE_SYSVSEM) {
> -		error = get_undo_list(&undo_list);
> +		error = get_undo_list(current, &undo_list);
>  		if (error)
>  			return error;
>  		atomic_inc(&undo_list->refcnt);
> @@ -1405,10 +1446,15 @@ next_entry:
>  	kfree(undo_list);
>  }
> 
> -/* called from do_exit() */
> +/* exit_sem: called from do_exit()
> + * task_lock is used to synchronize with get_undo_list()

Ok I had to think about this again.  I'd like the comment
here to point out that the task_lock here acts as a barrier
between the prior setting of PF_EXITING and the undo_list
being freed here, so that get_undo_list() will either see
PF_EXITING is NOT in the tsk->flags, in which case it will
insert the undo_list before the task_lock() is grabbed here,
and with count=2, so that it gets correctly put here in
exit_sem, or it will see PF_EXITING set and cancel the
undo_list it was creating.

> + */
>  void exit_sem(struct task_struct *tsk)
>  {
> -	struct sem_undo_list *ul = tsk->sysvsem.undo_list;
> +	struct sem_undo_list *ul;
> +	task_lock(tsk);
> +	ul = tsk->sysvsem.undo_list;
> +	task_unlock(tsk);
>  	if (ul) {
>  		rcu_assign_pointer(tsk->sysvsem.undo_list, NULL);
>  		synchronize_rcu();
> 
> -- 
> Pierre Peiffer
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