linux-kernel - Re: [PATCH] power: set PF_SUSPEND_TASK flag on tasks that call freeze

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Date:	Thu, 25 Jul 2013 22:06:46 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Colin Cross <ccross@...roid.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Michael Leun <lkml20130126@...ton.leun.net>,
	Pavel Machek <pavel@....cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Darren Hart <dvhart@...ux.intel.com>
Subject: Re: [PATCH] power: set PF_SUSPEND_TASK flag on tasks that call freeze_processes

On Wednesday, July 24, 2013 05:41:33 PM Colin Cross wrote:
> Calling freeze_processes sets a global flag that will cause any
> process that calls try_to_freeze to enter the refrigerator.  It
> skips sending a signal to the current task, but if the current
> task ever hits try_to_freeze all threads will be frozen and the
> system will deadlock.
> 
> Set a new flag, PF_SUSPEND_TASK, on the task that calls
> freeze_processes.  The flag notifies the freezer that the thread
> is involved in suspend and should not be frozen.  Also add a
> WARN_ON in thaw_processes if the caller does not have the
> PF_SUSPEND_TASK flag set to catch if a different task calls
> thaw_processes than the one that called freeze_processes, leaving
> a task with PF_SUSPEND_TASK permanently set on it.
> 
> Threads that spawn off a task with PF_SUSPEND_TASK set (which
> swsusp does) will also have PF_SUSPEND_TASK set, preventing them
> from freezing while they are helping with suspend, but they need
> to be dead by the time suspend is triggered, otherwise they may
> run when userspace is expected to be frozen.  Add a WARN_ON in
> thaw_processes if more than one thread has the PF_SUSPEND_TASK
> flag set.
> 
> Reported-by: Michael Leun <lkml20130126@...ton.leun.net>
> Tested-by: Michael Leun <lkml20130126@...ton.leun.net>
> Signed-off-by: Colin Cross <ccross@...roid.com>
> ---
> 
> Resending not as an attachment for review.

I like this, but I wonder what other people think.

> If the extra process flag is considered too precious for this
> (there are only 2 left after this patch) I could get the
> same functionality by having freeze_processes() reject calls
> from a PF_KTHREAD|PF_NOFREEZE thread, and use PF_KTHREAD to
> determine if PF_NOFREEZE should be cleared in thaw_processes().

Can we spend an extra process flag on that?

Rafael


>  include/linux/sched.h  |  1 +
>  kernel/freezer.c       |  2 +-
>  kernel/power/process.c | 11 +++++++++++
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 50d04b9..d722490 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1628,6 +1628,7 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
>  #define PF_MEMPOLICY	0x10000000	/* Non-default NUMA mempolicy */
>  #define PF_MUTEX_TESTER	0x20000000	/* Thread belongs to the rt mutex tester */
>  #define PF_FREEZER_SKIP	0x40000000	/* Freezer should not count it as freezable */
> +#define PF_SUSPEND_TASK 0x80000000      /* this thread called freeze_processes and should not be frozen */
>  
>  /*
>   * Only the _current_ task can read/write to tsk->flags, but other
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index 8b2afc1..b462fa1 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -33,7 +33,7 @@ static DEFINE_SPINLOCK(freezer_lock);
>   */
>  bool freezing_slow_path(struct task_struct *p)
>  {
> -	if (p->flags & PF_NOFREEZE)
> +	if (p->flags & (PF_NOFREEZE | PF_SUSPEND_TASK))
>  		return false;
>  
>  	if (pm_nosig_freezing || cgroup_freezing(p))
> diff --git a/kernel/power/process.c b/kernel/power/process.c
> index fc0df84..06ec886 100644
> --- a/kernel/power/process.c
> +++ b/kernel/power/process.c
> @@ -109,6 +109,8 @@ static int try_to_freeze_tasks(bool user_only)
>  
>  /**
>   * freeze_processes - Signal user space processes to enter the refrigerator.
> + * The current thread will not be frozen.  The same process that calls
> + * freeze_processes must later call thaw_processes.
>   *
>   * On success, returns 0.  On failure, -errno and system is fully thawed.
>   */
> @@ -120,6 +122,9 @@ int freeze_processes(void)
>  	if (error)
>  		return error;
>  
> +	/* Make sure this task doesn't get frozen */
> +	current->flags |= PF_SUSPEND_TASK;
> +
>  	if (!pm_freezing)
>  		atomic_inc(&system_freezing_cnt);
>  
> @@ -168,6 +173,7 @@ int freeze_kernel_threads(void)
>  void thaw_processes(void)
>  {
>  	struct task_struct *g, *p;
> +	struct task_struct *curr = current;
>  
>  	if (pm_freezing)
>  		atomic_dec(&system_freezing_cnt);
> @@ -182,10 +188,15 @@ void thaw_processes(void)
>  
>  	read_lock(&tasklist_lock);
>  	do_each_thread(g, p) {
> +		/* No other threads should have PF_SUSPEND_TASK set */
> +		WARN_ON((p != curr) && (p->flags & PF_SUSPEND_TASK));
>  		__thaw_task(p);
>  	} while_each_thread(g, p);
>  	read_unlock(&tasklist_lock);
>  
> +	WARN_ON(!(curr->flags & PF_SUSPEND_TASK));
> +	curr->flags &= ~PF_SUSPEND_TASK;
> +
>  	usermodehelper_enable();
>  
>  	schedule();
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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