SMP share data declaration

SMP share data declaration

[wyb]

I know that an integer variable should be declared volatile to share between
CPUs.
But for more complicated variable, I don't know if it should be volatile.
for example:

linux-2.6.6/kernel/workqueue.c:
struct workqueue_struct *__create_workqueue(const char *name,int
singlethread)
{
    ...
    if (singlethread) {
        ...
    } else {
        spin_lock(&workqueue_lock);
        list_add(&wq->list, &workqueues);
        spin_unlock_irq(&workqueue_lock);
        ...
    }
    ...
}

workqueues is a none volatile list_head. The spin_lock/unlock act as memory
barrier, so gcc flush any register cached value to memory. But if another
CPU allocate workqueues.next/prev in registers, how to make sure this cpu
get new value ?

static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
      unsigned long action,
      void *hcpu)
{
    ...
    switch (action) {
    case CPU_UP_PREPARE:
    /* Create a new workqueue thread for it. */
    list_for_each_entry(wq, &workqueues, list) {
        ...
}

how to make sure workqueue_cpu_callback() get new value ?

thanks very much

Re: SMP share data declaration

[Jesper Juhl]

On 14/07/06, wyb@topsec.com.cn <wyb@topsec.com.cn> wrote:
> I know that an integer variable should be declared volatile to share between
> CPUs.

NO. volatile won't protect you sufficiently.

Use spinlocks, mutexes, semaphores, barriers and the like to protect
variables from concurrent access. Using volatile for this is a BUG and
it won't work correctly.


-- 
Jesper Juhl <jesper.juhl@gmail.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html

RE: SMP share data declaration

[Hannibal B]

Spinlocks would be the one to protect the variable and synchronies
between CPU, But it's a busy waiting mechanism(just a word of caution).
Regards,
Hannibal

-----Original Message-----
From: Jesper Juhl [mailto:jesper.juhl@gmail.com] 
Sent: Friday, July 14, 2006 2:18 PM
To: wyb@topsec.com.cn
Cc: linux-kernel@vger.kernel.org
Subject: Re: SMP share data declaration

On 14/07/06, wyb@topsec.com.cn <wyb@topsec.com.cn> wrote:
> I know that an integer variable should be declared volatile to share
between
> CPUs.

NO. volatile won't protect you sufficiently.

Use spinlocks, mutexes, semaphores, barriers and the like to protect
variables from concurrent access. Using volatile for this is a BUG and
it won't work correctly.


-- 
Jesper Juhl <jesper.juhl@gmail.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/