linux-kernel - Re: Process id recycling and status of tasks

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Message-ID: <f22d86810910241107l21159ccdx2203e2b537b7f36@mail.gmail.com>
Date:	Sat, 24 Oct 2009 23:37:22 +0530
From:	"Leonidas ." <leonidas137@...il.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Process id recycling and status of tasks

On Fri, Oct 23, 2009 at 1:43 PM, Clemens Ladisch <clemens@...isch.de> wrote:
> Leonidas . wrote:
>> 1. What would be an ideal way to check if a task is alive or dead from
>> kernel space?
>
> Get a reference to the task's pid (call get_task_pid(), or get_pid() on
> the return value of task_pid()), then later check whether pid_task()
> works.
>
> (pid_alive() is not what you want because this would require that you
> hold a reference to the task_struct.)
>
>> even though a certain pid might exist, it might have got recycled by
>> the time I check again, right?
>
> The functions above do not work with PID numbers but with struct pid
> which is a reference-counted object.  (The functions with "get" in their
> name increase the reference count, so don't forget to put_pid() when you
> no longer need it.)  See also the big comment in include/linux/pid.h.
>
>
> HTH
> Clemens
>

Yes, the comment in pid.h says it.

Was going through pid.c, what is the fundamental difference between pid_task()
and get_pid_task()? Is it correct to say that, get_pid_task() will
check whether the
task struct is stale or not and return accordingly and pid_task() will
blindly return
task_struct which might be stale?

Now my understanding is get_pid_task() should be followed by put_pid_task()
so the reference counting work as expected, but put_pid_task() is not an
exported symbol? Am I missing here something?

The obvious question which follows from above is what would be the correct
way to determine whether a process is alive or not using pid_alive()? Using
pid_task() does not seem correct and seemingly correct way looks unfeasible.

-Leo.
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