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

[Pierre Peiffer]

Serge E. Hallyn wrote:
> Quoting pierre.peiffer@xxxxxxxx (pierre.peiffer@xxxxxxxx):
>> From: Pierre Peiffer <pierre.peiffer@xxxxxxxx>
>>
>> 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@xxxxxxxx>
>> ---
>>
>>  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?
> 

I inc it  outside this function, as I don't call get_undo_list() if there is an
undo_list.
This appears most clearly in the next patch, in semundo_open() for example.

>> -	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.
> 

Yep, I will add this to clarify this point.

Thanks Serge.

P.

>> + */
>>  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
>> _______________________________________________
>> Containers mailing list
>> Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx
>> https://lists.linux-foundation.org/mailman/listinfo/containers
> 
> 

-- 
Pierre Peiffer
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