Re: [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify the logic in lookup_task

Oleg Nesterov
2020-04-26T17:22:22Z

Eric,

I am sick today and can't read the code, but I feel this patch is not
right ... please correct me.

So, iiuc when posix_cpu_timer_create() is called and CPUCLOCK_PERTHREAD
is false we roughly have

task = pid_task(pid, PIDTYPE_TGID); // lookup_task()

/* WINDOW */

timer->it.cpu.pid = = get_task_pid(task, PIDTYPE_TGID) // posix_cpu_timer_create()

Now suppose that we race with mt-exec and this "task" is the old leader;
it can be release_task()'ed in the WINDOW above and then get_task_pid()
will return NULL.

That is why I suggested to change lookup_task() to return "struct pid*"
to eliminate the pid -> task -> pid transition.

Apart from the same_thread_group() check for the "thread" case we do not
need task_struct at all, lookup_task() can do

if (thread) {
p = pid_task(pid, PIDTYPE_PID);
if (p && !same_thread_group(p, current))
pid = NULL;
} else {
... gettime check ...

if (!pid_has_task(pid, PIDTYPE_TGID))
pid = NULL;
}

return pid;

No?

Oleg.