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SubjectRe: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held
On 04/26, Eric W. Biederman wrote:
>
> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
> }
>
> sighand = parent->sighand;
> - spin_lock_irqsave(&sighand->siglock, flags);
> + lock = tsk->sighand != sighand;
> + if (lock)
> + spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING);

But why is it safe?

Suppose we have two tasks, they both trace each other, both call
ptrace_stop() at the same time. Of course this is ugly, they both
will block.

But with this patch in this case we have the trivial ABBA deadlock,
no?

Oleg.

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