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

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Message-Id: <1191421736.5599.18.camel@lappy>
Date:	Wed, 03 Oct 2007 16:28:56 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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.


On Wed, 2007-10-03 at 23:19 +0900, Tetsuo Handa wrote:
> 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);
> 
> }

If at all possible, use struct mutex.

> read_function() {
> 
>   for (ptr = head; ptr; ptr = ptr->next) {
>     ...
>   }
> 
> }
> 
> Are (4) and (5) needed even when (3) is exclusively protected by down() and up() ?

the up() would do 4. 5 ensures another cpu will actually see it. Althoug
in practise the various cache invalidations driven by the workload will
ensure it will become visible eventually anyway.

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