Re: [PATCH 2.6.24-rc8-mm1 12/15] (RFC) IPC/semaphores: make use of RCU to free the sem_undo_list

[Pierre Peiffer <pierre.peiffer@bull.net>]

Serge E. Hallyn wrote:
> Quoting pierre.peiffer@bull.net (pierre.peiffer@bull.net):
>> From: Pierre Peiffer <pierre.peiffer@bull.net>
>>
>> Today, the sem_undo_list is freed when the last task using it exits.
>> There is no mechanism in place, that allows a safe concurrent access to
>> the sem_undo_list of a target task and protects efficiently against a
>> task-exit.
>>
>> That is okay for now as we don't need this.
>>
>> As I would like to provide a /proc interface to access this data, I need
>> such a safe access, without blocking the target task if possible. 
>>
>> This patch proposes to introduce the use of RCU to delay the real free of
>> these sem_undo_list structures. They can then be accessed in a safe manner
>> by any tasks inside read critical section, this way:
>>
>> 	struct sem_undo_list *undo_list;
>> 	int ret;
>> 	...
>> 	rcu_read_lock();
>> 	undo_list = rcu_dereference(task->sysvsem.undo_list);
>> 	if (undo_list)
>> 		ret = atomic_inc_not_zero(&undo_list->refcnt);
>> 	rcu_read_unlock();
>> 	...
>> 	if (undo_list && ret) {
>> 		/* section where undo_list can be used quietly */
>> 		...
>> 	}
>> 	...
> 
> And of course then
> 
> 	if (atomic_dec_and_test(&undo_list->refcnt))
> 		free_semundo_list(undo_list);
> 
> by that task.
> 

I will precise this too.

>> Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>
> 
> Looks correct in terms of locking/refcounting.
> 
> Signed-off-by: Serge Hallyn <serue@us.ibm.com>
> 

Thanks !

-- 
Pierre