Messages in this thread | | | Date | Fri, 12 Jun 2009 16:00:44 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] posix-cpu-timers: remove tasklist_lock where we can |
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On 06/12, Stanislaw Gruszka wrote: > > tasklist_lock is not needed to protect find_task_by_vpid() nor > thread_group_leader()
It does protect thread_group_leader(), unless we use current. Please see below.
> @@ -398,7 +398,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > new_timer->it.cpu.incr.sched = 0; > new_timer->it.cpu.expires.sched = 0; > > - read_lock(&tasklist_lock); > + rcu_read_lock(); > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) { > if (pid == 0) { > p = current; > @@ -422,7 +422,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > } else { > ret = -EINVAL; > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock();
Suppose that the non-main thread execs. de_thread() does
attach_pid(tsk, PIDTYPE_PID, task_pid(leader)); leader->group_leader = tsk;
under write_lock(tasklist). This means posix_cpu_timer_create() can return -EINVAL if it is called in between, or if we found the group leader but thread_group_leader() is called after "tsk->group_leader = tsk".
I think the patch is fine, but you should also replace thread_group_leader() with has_group_leader_pid().
This reminds me. !CPUCLOCK_PERTHREAD timers survive after exec, but only if ->group_leader execs. Not good.
Oleg.
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