linux-kernel - Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

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Date:   Thu, 9 Apr 2020 21:57:33 +0200
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

On 4/9/20 9:42 PM, Linus Torvalds wrote:
> On Thu, Apr 9, 2020 at 11:36 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> I guess I need to look at what that test is actually testing, because
>> it wasn't what I thought.
> 
> Ahh.
> 
> The problem is that zap_other_threads() counts all threads.
> 
> But it doesn't bother notifying already dead threads, even if it counts them.
> 
> And then it waits for the threads to go away, but didn't do anything
> to make that dead thread go away.
> 
> And the test case has an already dead thread that is just waiting to
> be reaped by the same person who is now waiting for it to go away.
> 
> So it just stays around.
> 
> Honestly, I'm not entirely sure this is worth worrying about, since
> it's all killable anyway and only happens if you do something stupid.
> 

The use case where this may happen with strace
when you call strace with lots of -p <pid> arguments,
and one of them is a bomb. strace stuck.

So when that happens in the beginning, it is not much
work lost, but if you traced a megabyte of data to analyze
and then that happens, you are not really amused.

Also slightly different things happen with PTRACE_O_TRACEEXIT
then the tracer is supposed to continue the exit, and then
to wait for the thread to die.  Which is twice as ugly...


Bernd.


> I mean, you can get two threads to wait for each other more easily other ways.
> 
> Or maybe we just shouldn't count already dead threads? Yeah, they'd
> share that current signal struct, but they're dead and can't do
> anything about it, they can only be reaped.
> 
> But that would mean that we should also move the signal->notify_count
> update to when we mark the EXIT_ZOMBIE or EXIT_DEAD in exit_state.
> 
>           Linus
>