Messages in this thread | | | From | "Eric W. Biederman" <> | Date | Thu, 21 Apr 2022 13:40:57 -0500 | Subject | Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT |
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Peter Zijlstra <peterz@infradead.org> writes:
> Rework ptrace_check_attach() / ptrace_unfreeze_traced() to not rely on > task->__state as much. > > Due to how PREEMPT_RT is changing the rules vs task->__state with the > introduction of task->saved_state while TASK_RTLOCK_WAIT (the whole > blocking spinlock thing), the way ptrace freeze tries to do things no > longer works.
The problem with ptrace_stop and do_signal_stop that requires dropping siglock and grabbing tasklist_lock is that do_notify_parent_cldstop needs tasklist_lock to keep parent and real_parent stable.
With just some very modest code changes it looks like we can use a processes own siglock to keep parent and real_parent stable. The siglock is already acquired in all of those places it is just not held over the changing parent and real_parent.
Then make a rule that a child's siglock must be grabbed before a parents siglock and do_notify_parent_cldstop can be always be called under the childs siglock.
This means ptrace_stop can be significantly simplified, and the notifications can be moved far enough up that set_special_state can be called after do_notify_parent_cldstop. With the result that there is simply no PREEMPT_RT issue to worry about and wait_task_inactive can be used as is.
I remember Oleg suggesting a change something like this a long time ago.
I need to handle the case where the parent and the child share the same sighand but that is just remembering to handle it in do_notify_parent_cldstop, as the handling is simply not taking the lock twice.
I am going to play with that and see if I there are any gotcha's I missed when looking through the code.
Eric
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