Oleg Nesterov <[email protected]> writes:
> (COMPILE TESTED, needs an ack from Eric)
>
> proc_pid_make_inode:
>
> ei->pid = get_pid(task_pid(task));
>
> I think this is not safe. get_pid() can be preempted after checking
> "pid != NULL". Then the task exits, does detach_pid(), and RCU frees
> the pid.
Ugh. I had forgotten that the pid of a task gets freed even if you
hold a reference to the task struct. So the preemption case looks possible.
Your technique to handle this problem looks fine.
As for the functions can we build them in all 4 varieties.
struct pid *get_task_pid(struct task *);
struct pid *get_task_tgid(struct task *);
struct pid *get_task_pgrp(struct task *);
struct pid *get_task_session(struct task *);
Functions without a flag are less error prone to use, and clearer to read.
Either that or we can just drop in some rcu_read_lock() rcu_read_unlock()
into the call sites.
Eric
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