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SubjectRe: [PATCH] proc: readdir race fix
On 09/04, Eric W. Biederman wrote:
>
> -static struct task_struct *next_tgid(struct task_struct *start)
> -{
> - struct task_struct *pos;
> + task = NULL;
> rcu_read_lock();
> - pos = start;
> - if (pid_alive(start))
> - pos = next_task(start);
> - if (pid_alive(pos) && (pos != &init_task)) {
> - get_task_struct(pos);
> - goto done;
> +retry:
> + pid = find_next_pid(tgid);
> + if (pid) {
> + tgid = pid->nr + 1;
> + task = pid_task(pid, PIDTYPE_PID);
> + if (!task || !thread_group_leader(task))
^^^^^^^^^^^^^^^^^^^
There is a window while de_thread() switches leadership, so next_tgid()
may skip a task doing exec. What do you think about

// needs a comment
if (!task || task->pid != task->tgid)
goto retry;

instead? Currently first_tgid() has the same (very minor) problem.

> + goto retry;
> + get_task_struct(task);
> }
> - pos = NULL;
> -done:
> rcu_read_unlock();
> - put_task_struct(start);
> - return pos;
> + return task;
> +
> }

Emply line before '}'

> +struct pid *find_next_pid(int nr)
> +{
> + struct pid *next;
> +
> + next = find_pid(nr);
> + while (!next) {
> + nr = next_pidmap(nr);
> + if (nr <= 0)
> + break;
> + next = find_pid(nr);
> + }
> + return next;
> +}

This is strange that we are doing find_pid() before and at the end of loop,
I'd suggest this code:

struct pid *find_next_pid(int nr)
{
struct pid *pid;

do {
pid = find_pid(nr);
if (pid != NULL)
break;
nr = next_pidmap(nr);
} while (nr > 0);

return pid;
}

Imho, a bit easier to read.

Oleg.

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