On Wed 01-06-16 00:29:33, Oleg Nesterov wrote:

On 05/31, Michal Hocko wrote:

quoted

On Mon 30-05-16 19:35:05, Oleg Nesterov wrote:

quoted

Well, let me suggest this again. I think it should do


	if (SIGNAL_GROUP_COREDUMP)
		return false;

	if (SIGNAL_GROUP_EXIT)
		return true;

	if (thread_group_empty() && PF_EXITING)
		return true;

	return false;

we do not need fatal_signal_pending(), in this case SIGNAL_GROUP_EXIT should
be set (ignoring some bugs with sub-namespaces which we need to fix anyway).

OK, so we shouldn't care about race when the fatal_signal is set on the
task until it reaches do_group_exit?

if fatal_signal() is true then (ignoring exec and coredump) SIGNAL_GROUP_EXIT
is already set (again, ignoring the bugs with sub-namespace inits).

At the same time, SIGKILL can be already dequeued when the task exits, so
fatal_signal_pending() can be "false negative".

Thanks for the clarification. I guess I got the point but this is a land
of surprises so one can never be sure...

quoted

quoted

And. I think this needs smp_rmb() at the end of the loop (assuming we have the
process_shares_mm() check here). We need it to ensure that we read p->mm before
we read next_task(), to avoid the race with exit() + clone(CLONE_VM).

Why don't we need the same barrier in oom_kill_process?

Because it calls do_send_sig_info() which takes ->siglock and copy_process()
takes the same lock. Not a barrier, but acts the same way.

Ahh ok, so an implicit barrier.

quoted

Which barrier it
would pair with?

With the barrier implied by list_add_tail_rcu(&p->tasks, &init_task.tasks).

Ahh I see. rcu_assign_pointer that is, right?

quoted

Anyway I think this would deserve it's own patch.
Barriers are always tricky and it is better to have them in a small
patch with a full explanation.

OK, agreed.

cool

I am not sure I can read the new patch correctly, it depends on the previous
changes... but afaics it looks good.

Cosmetic/subjective nit, feel free to ignore,

quoted

+bool task_will_free_mem(struct task_struct *task)
+{
+	struct mm_struct *mm = NULL;

unnecessary initialization ;)

fixed

quoted

+	struct task_struct *p;
+	bool ret;
+
+	/*
+	 * If the process has passed exit_mm we have to skip it because
+	 * we have lost a link to other tasks sharing this mm, we do not
+	 * have anything to reap and the task might then get stuck waiting
+	 * for parent as zombie and we do not want it to hold TIF_MEMDIE
+	 */
+	p = find_lock_task_mm(task);
+	if (!p)
+		return false;
+
+	if (!__task_will_free_mem(p)) {
+		task_unlock(p);
+		return false;
+	}

We can call the 1st __task_will_free_mem(p) before find_lock_task_mm(). In the
likely case (I think) it should return false.

OK

And since __task_will_free_mem() has no other callers perhaps it should go into
oom_kill.c too.

ok

I will resend the whole series with the fixups later during this week.
Thanks again for your review.
-- 
Michal Hocko
SUSE Labs

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