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Subject: Re: Syscall kill() can send signal to thread ID
From:   Cambda Zhu <cambda@linux.alibaba.com>
In-Reply-To: <87illeedc5.fsf@oldenburg.str.redhat.com>
Date:   Fri, 23 Sep 2022 16:40:15 +0800
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Dust Li <dust.li@linux.alibaba.com>,
        Tony Lu <tonylu@linux.alibaba.com>
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To:     Florian Weimer <fweimer@redhat.com>
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> On Sep 23, 2022, at 15:53, Florian Weimer <fweimer@redhat.com> wrote:
> 
>> I don't quite understand what you mean, sorry. But if kill() returns
>> -ESRCH for tid which is not equal to tgid, kill() can only send signal
>> to thread group via main thread id, that is what BSD did and manual
>> said. It seems not odd?
> 
> It's still odd because there's one TID per process that's valid for
> kill by accident.  That's all.
> 
> Thanks,
> Florian

As far as I know, there is no rule forbidding 'process ID'(TGID on Linux)
equals to main thread ID, is it right? If one wants to send signal to a
specific thread, tgkill() can do that. As far as I understand, the difference
between kill() and tgkill() is whether the signal is set on shared_pending,
whatever the ID is a process ID or a thread ID. For Linux, the main thread ID
just equals to the process ID. So the meaning of kill(main_tid, sig) is sending
signal to a process, of which the PID equals to the first argument. It's not odd,
I think.

Thanks,
Cambda