Serge E. Hallyn wrote:
> Quoting pierre.peiffer@xxxxxxxx (pierre.peiffer@xxxxxxxx):
>> From: Pierre Peiffer <pierre.peiffer@xxxxxxxx>
>>
>> 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@xxxxxxxx>
>
> Looks correct in terms of locking/refcounting.
>
> Signed-off-by: Serge Hallyn <serue@xxxxxxxxxx>
>
Thanks !
--
Pierre
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