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

[Gautham R Shenoy <ego@in.ibm.com>]

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!"