linux-kernel - Re: [TOMOYO 05/15](repost) Domain transition handler functions.

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Message-Id: <200710032319.ABF69743.VJQMLHFOFOtFOS@I-love.SAKURA.ne.jp>
Date:	Wed, 3 Oct 2007 23:19:20 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	a.p.zijlstra@...llo.nl
Cc:	jmorris@...ei.org, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, chrisw@...s-sol.org
Subject: Re: [TOMOYO 05/15](repost) Domain transition handler functions.

Hello.

Thank you for pointing out.

Peter Zijlstra wrote:
> > Currently, TOMOYO Linux avoids read_lock, on the assumption that
> > (1) First, ptr->next is initialized with NULL.
> > (2) Later, ptr->next is assigned non-NULL address.
> > (3) Assigning to ptr->next is done atomically.
>  (4) wmb after asigning ptr->next
>  (5) rmb before reading ptr->next
Excuse me, but I didn't understand why (4) and (5) are needed.

append_function() {

  down(semaphore_for_write_protect);
  ...
  ptr = head;
  while (ptr->next) ptr = ptr->next;
  ptr->next = new_entry;
  ...
  up(semaphore_for_write_protect);

}

read_function() {

  for (ptr = head; ptr; ptr = ptr->next) {
    ...
  }

}

Are (4) and (5) needed even when (3) is exclusively protected by down() and up() ?

Regards.

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