linux-kernel - Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

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Message-ID: <6599ad830908041228w67bc6f7fh57e28f244e1923b3@mail.gmail.com>
Date:	Tue, 4 Aug 2009 12:28:48 -0700
From:	Paul Menage <menage@...gle.com>
To:	Benjamin Blum <bblum@...gle.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid 
	at once

On Tue, Aug 4, 2009 at 12:14 PM, Benjamin Blum<bblum@...gle.com> wrote:
>
> Hmm, the tasklist_lock section in fork() is entirely inside the
> read-lock. Presumably then iterating the threadgroup list to take all
> rwsems is safe from a race in which one thread escapes?
>

I think so - the write-side locking would be something like:

again:
  read_lock(&tasklist_lock);
  find first not-yet-locked task t
  while (t != group_leader) {
    if (!down_write_trylock(&t->threadgroup_stability_mutex)) {
      get_task_struct(t);
      read_unlock(&tasklist_lock);
      down_write(&t->threadgroup_stability_mutex);
      put_task_struct(t);
      goto again;
    }
    t = t->next_thread;
  }
  read_unlock(&tasklist_lock);

  Paul
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