linux-kernel - Process id recycling and status of tasks

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Message-ID: <f22d86810910220629o433ee5d0u9857d1fb1f01cd6@mail.gmail.com>
Date:	Thu, 22 Oct 2009 06:29:39 -0700
From:	"Leonidas ." <leonidas137@...il.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Process id recycling and status of tasks

Hello,

1. What would be an ideal way to check if a task is alive or dead from
kernel space?
I dont want to check current execution state of the task, rather I
want to check whether task
with this pid is existing currently or not?  I might want to do it
periodically, hence even though
a certain pid might exist, it might have got recycled by the time I
check again, right?

Following is an elementary attempt to achieve the same, but I am not
convinced much, and
I am not even sure I can do this kind of thing without knowing the
context I am executing in.

task_t *p;
read_lock(&tasklist_lock);
for_each_process(p) {
 if ( strcmp (p->comm, $your-daemon-name) == 0)
   break;
}
read_unlock(&tasklist_lock);

2. Basically, what would be parameters of a task which will uniquely
identify a task which ever
existed on my machine. Would it be <name, pid> tuple? Or something
else which uniquely
does this job? Basically differentiate N user space threads in a process.

3. What is the algorithm followed for recycling pids? It does not look
very straight fwd like
pick up the latest freed pid for new task etc. I guess the pid
recycling must be starting after
certain threshold.

Any pointers will be helpful.

-Leo.
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