Re: [PATCH] sys_getppid oopses on debug kernel (v2)

From: Oleg Nesterov
Date: Wed Aug 09 2006 - 06:12:14 EST

Andrew Morton wrote:
> Although I'm not sure it's needed for this problem. A getppid() which does
> asmlinkage long sys_getppid(void)
> {
> int pid;
> read_lock(&tasklist_lock);
> pid = current->group_leader->real_parent->tgid;
> read_unlock(&tasklist_lock);
> return pid;
> }
> seems like a fine implementation to me ;)

Why do we need to use ->group_leader? All threads should have the same

Why do we need tasklist_lock? I think rcu_read_lock() is enough.

In other words, do you see any problems with this code

smlinkage long sys_getppid(void)
int pid;

pid = rcu_dereference(current->real_parent)->tgid;

return pid;

? Yes, we may read a stale value for ->real_parent, but the memory
can't be freed while we are under rcu_read_lock(). And in this case
the returned value is ok because the task could be reparented just
after return anyway.


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