Re: [RFC PATCH(experimental) 2/2] Fix freezer-kthread_stop race

Gautham R Shenoy
2007-04-20T11:05:26Z

On Fri, Apr 20, 2007 at 10:54:36AM +0200, Rafael J. Wysocki wrote:
>
> Hmm, can't we do something like this instead:
>
> ---
> kernel/kthread.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> Index: linux-2.6.21-rc7/kernel/kthread.c
> ===================================================================
> --- linux-2.6.21-rc7.orig/kernel/kthread.c
> +++ linux-2.6.21-rc7/kernel/kthread.c
> @@ -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 +233,15 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> + if (!(current->flags & PF_NOFREEZE)) {
> + /* If we are freezable, the freezer will wait for us */
> + task_lock(k);
> + k->flags |= PF_NOFREEZE;
> + if (frozen(k))
> + k->flags &= ~PF_FROZEN;
> +
> + task_unlock(k);
> + }

Yes, we can do this for now since the tasks have only two freeze states,
namely Freezeable and Non Freezeable.
But with more events like cpu-hotplug and kprobes using the process
freezer, the simple check

if(!(current->flags & PF_NOFREEZE))

may not suffice and something like
if(!(current->flags & PFE_ALL & global_freeze_mask))
/* global_freeze_mask is the mask of the events for which the system
is freezing. */

appears to be racy. Not that we cannot work it out ;-)

However, I was attempting to solve the generic problem where
A waits on B and B is frozen. If A is freezeable (under one of the
events) then the freezer will fail. So a solution would be for A to
somehow inform B of the dependency and postpone it's freezing.

Since akpm mentioned that flush_workqueue() needs to go, I guess, I am
ok with fixing only kthread_stop/kthread_should_stop for the moment.
Unless I can spot any other valid case :)

> wake_up_process(k);
> put_task_struct(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!"