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

Peter Zijlstra
2007-10-03T14:34:52Z

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.