From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)
Subject: Re: [PATCH 0/2] fix the traced mt-exec deadlock
Date: Fri, 03 Mar 2017 14:06:30 -0600
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ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) writes:

> ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) writes:
>
>> The big lesson for me, and what was not obvious from your change
>> description is that we are changing the user space visible semantics
>> of exec+ptrace and that cred_guard_mutex is not at all the problem (as
>> we always take cred_guard_mutex in a killable or interruptible way).
>
> Just to follow up.
>
> Because the cred_guard_mutex is fine as is we don't need to move
> de_thread out from under cred_guard_mutex.  We just need to change
> de_thread to wait until all of the other threads are zombies.
> Which should remove about half your proposed patch.
>
> The other key thing is that knowning it isn't cred_guard_mutex let's us
> know that this kind of deadlock goes all of the way back to when
> CLONE_THREAD was merged into the kernel.
>
> Insteresingly enough looking at zap_other_threads and notify_count I
> have found a second bug.  When a multi-threaded processes becomes a
> zombie we don't send the notification to the parent process until the
> non-leader threads have been reaped.  Which means ptrace can mess up
> sending SIGCHLD to the parent.

Bah. I was misreading the code.  Nothing but exec uses notify_count
and group_exit_task.

Eric