Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop

Gautham R Shenoy
2007-04-23T12:38:08Z

On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rjw@sisk.pl>
>
> Fix the problem with kthread_stop() that causes the freezer to fail if a
> freezable thread is attempting to stop a frozen one and that may cause the
> freezer to fail if the thread being stopped is freezable and
> try_to_freeze_tasks() is running concurrently with kthread_stop().
>
> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
> ---
> kernel/kthread.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> ===================================================================
> --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c 2007-04-09 15:23:48.000000000 +0200
> +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.000000000 +0200
> @@ -13,6 +13,7 @@
> #include <linux/file.h>
> #include <linux/module.h>
> #include <linux/mutex.h>
> +#include <linux/freezer.h>
> #include <asm/semaphore.h>
>
> /*
> @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> + if (!freezer_should_exempt(current)) {
> + /* We are freezable, so we must make sure that the thread being
> + * stopped is not frozen and will not be frozen until it dies
> + */
> + freezer_exempt(k);
> + if (frozen(k))
> + clear_frozen_flag(k);
> + }

I'm trying hard to convince myself that this will work. May be I am
missing something here, but I find a potential race window (very small though)
when k is entering the refrigerator.

Here's how.

kthread_stop(k) k->refrigerator()
---------------------------------------------------------------------
task_lock(k);
1) check_if_exempted();
/* not exempted. So
* we will freeze
* ourself.
*/
2) freezer_exempt(k);

3) if(frozen(k))
/* No, we're not yet frozen. So we
* don't clear_frozen_flag(k) here
*/
4) frozen_process(k);
task_unlock(k);

5) for(;;) {
set_current_state(UNINTERRUPTIBLE);
if(!frozen_process(k))
/* k is frozen. We
* don't break :(
*/

schedule();
}

> wake_up_process(k);
> put_task_struct(k);
>

Thus the freezer can still fail, no?
IMO, we need the to take the task_lock for k here. Something like

> + if (!freezer_should_exempt(current)) {
task_lock(k);
> + /* We are freezable, so we must make sure that the thread being
> + * stopped is not frozen and will not be frozen until it dies
> + */
> + freezer_exempt(k);
> + if (frozen(k))
> + clear_frozen_flag(k);
task_unlock(k);
> + }

Thanks and Regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"