Date:	Wed, 7 May 2008 14:11:57 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	ego@...ibm.com
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cedric Le Goater <clg@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Menage <menage@...gle.com>
Subject: Re: [linux-pm] [RFC][PATCH 1/2] Freezer: Introduce PF_FREEZER_NOSIG

On Wednesday, 7 of May 2008, Gautham R Shenoy wrote:
> Hi Rafael,

Hi,

> On Wed, May 07, 2008 at 12:05:19AM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rjw@...k.pl>
> > 
> > The freezer currently attempts to distinguish kernel threads from
> > user space tasks by checking if their mm pointer is unset and it
> > does not send fake signals to kernel threads.  However, there are
> > kernel threads, mostly related to networking, that behave like
> > user space tasks and may want to be sent a fake signal to be frozen.
> > 
> > Introduce the new process flag PF_FREEZER_NOSIG that will be set
> > by default for all kernel threads and make the freezer only send
> > fake signals to the tasks having PF_FREEZER_NOSIG unset.  Provide
> > the set_freezable_with_signal() function to be called by the kernel
> > threads that want to be sent a fake signal for freezing.
> > 
> > This patch should not change the freezer's observable behavior.
> > 
> > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> > ---
> >  include/linux/freezer.h |   10 ++++
> >  include/linux/sched.h   |    1 
> >  kernel/kthread.c        |    2 
> >  kernel/power/process.c  |   97 ++++++++++++++++++++----------------------------
> >  4 files changed, 54 insertions(+), 56 deletions(-)
> > 
> > Index: linux-2.6/include/linux/freezer.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/freezer.h
> > +++ linux-2.6/include/linux/freezer.h
> > @@ -128,6 +128,15 @@ static inline void set_freezable(void)
> >  }
> > 
> >  /*
> > + * Tell the freezer that the current task should be frozen by it and that it
> > + * should send a fake signal to the task to freeze it.
> > + */
> > +static inline void set_freezable_with_signal(void)
> > +{
> > +	current->flags &= ~(PF_NOFREEZE | PF_FREEZER_NOSIG);
> > +}
> > +
> > +/*
> >   * Freezer-friendly wrappers around wait_event_interruptible() and
> >   * wait_event_interruptible_timeout(), originally defined in <linux/wait.h>
> >   */
> > @@ -174,6 +183,7 @@ static inline void freezer_do_not_count(
> >  static inline void freezer_count(void) {}
> >  static inline int freezer_should_skip(struct task_struct *p) { return 0; }
> >  static inline void set_freezable(void) {}
> > +static inline void set_freezable_with_signal(void) {}
> > 
> >  #define wait_event_freezable(wq, condition)				\
> >  		wait_event_interruptible(wq, condition)
> > Index: linux-2.6/include/linux/sched.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/sched.h
> > +++ linux-2.6/include/linux/sched.h
> > @@ -1508,6 +1508,7 @@ static inline void put_task_struct(struc
> >  #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 freezeable */
> > +#define PF_FREEZER_NOSIG 0x80000000	/* Freezer won't send signals to it */
> > 
> >  /*
> >   * Only the _current_ task can read/write to tsk->flags, but other
> > Index: linux-2.6/kernel/power/process.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/power/process.c
> > +++ linux-2.6/kernel/power/process.c
> > @@ -19,9 +19,6 @@
> >   */
> >  #define TIMEOUT	(20 * HZ)
> > 
> > -#define FREEZER_KERNEL_THREADS 0
> > -#define FREEZER_USER_SPACE 1
> > -
> >  static inline int freezeable(struct task_struct * p)
> >  {
> >  	if ((p == current) ||
> > @@ -84,63 +81,53 @@ static void fake_signal_wake_up(struct t
> >  	spin_unlock_irqrestore(&p->sighand->siglock, flags);
> >  }
> > 
> > -static int has_mm(struct task_struct *p)
> > +static inline bool should_send_signal(struct task_struct *p)
> >  {
> > -	return (p->mm && !(p->flags & PF_BORROWED_MM));
> > +	return !(current->flags & PF_FREEZER_NOSIG);
> 		^^^^^^^^^^^^^^
> 		p->flags ?

Yes, nice catch, thanks!

I wonder how on Earth that wasn't caught by testing (current->flags is always
false here) ...

[--snip--]
> > 
> > -static void thaw_tasks(int thaw_user_space)
> > +static void thaw_tasks(bool sig_only)
> >  {
> >  	struct task_struct *g, *p;
> > 
> > @@ -260,7 +247,7 @@ static void thaw_tasks(int thaw_user_spa
> >  		if (!freezeable(p))
> >  			continue;
> > 
> > -		if (!p->mm == thaw_user_space)
> > +		if (sig_only && !should_send_signal(p))
> >  			continue;
> > 
> >  		thaw_process(p);
> > @@ -271,8 +258,8 @@ static void thaw_tasks(int thaw_user_spa
> >  void thaw_processes(void)
> >  {
> >  	printk("Restarting tasks ... ");
> > -	thaw_tasks(FREEZER_KERNEL_THREADS);
> > -	thaw_tasks(FREEZER_USER_SPACE);
> > +	thaw_tasks(true);
> > +	thaw_tasks(false);
> 
> Shouldn't the order be 
> 	thaw_tasks(false);
> 	thaw_tasks(true);

Ouch, thanks again!

But, that's supposed to be "first thaw tasks we did not send fake signals to
and then thaw everybody else", which translates to something different from
what I wrote in thaw_tasks(), too.

[Well, frankly I'm not sure if there is a practical situation in which that
matters.]

Corrected patch is appended.

Thanks,
Rafael


---
 include/linux/freezer.h |   10 ++++
 include/linux/sched.h   |    1 
 kernel/kthread.c        |    2 
 kernel/power/process.c  |   97 ++++++++++++++++++++----------------------------
 4 files changed, 54 insertions(+), 56 deletions(-)

Index: linux-2.6/include/linux/freezer.h
===================================================================
--- linux-2.6.orig/include/linux/freezer.h
+++ linux-2.6/include/linux/freezer.h
@@ -128,6 +128,15 @@ static inline void set_freezable(void)
 }
 
 /*
+ * Tell the freezer that the current task should be frozen by it and that it
+ * should send a fake signal to the task to freeze it.
+ */
+static inline void set_freezable_with_signal(void)
+{
+	current->flags &= ~(PF_NOFREEZE | PF_FREEZER_NOSIG);
+}
+
+/*
  * Freezer-friendly wrappers around wait_event_interruptible() and
  * wait_event_interruptible_timeout(), originally defined in <linux/wait.h>
  */
@@ -174,6 +183,7 @@ static inline void freezer_do_not_count(
 static inline void freezer_count(void) {}
 static inline int freezer_should_skip(struct task_struct *p) { return 0; }
 static inline void set_freezable(void) {}
+static inline void set_freezable_with_signal(void) {}
 
 #define wait_event_freezable(wq, condition)				\
 		wait_event_interruptible(wq, condition)
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1508,6 +1508,7 @@ static inline void put_task_struct(struc
 #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 freezeable */
+#define PF_FREEZER_NOSIG 0x80000000	/* Freezer won't send signals to it */
 
 /*
  * Only the _current_ task can read/write to tsk->flags, but other
Index: linux-2.6/kernel/power/process.c
===================================================================
--- linux-2.6.orig/kernel/power/process.c
+++ linux-2.6/kernel/power/process.c
@@ -19,9 +19,6 @@
  */
 #define TIMEOUT	(20 * HZ)
 
-#define FREEZER_KERNEL_THREADS 0
-#define FREEZER_USER_SPACE 1
-
 static inline int freezeable(struct task_struct * p)
 {
 	if ((p == current) ||
@@ -84,63 +81,53 @@ static void fake_signal_wake_up(struct t
 	spin_unlock_irqrestore(&p->sighand->siglock, flags);
 }
 
-static int has_mm(struct task_struct *p)
+static inline bool should_send_signal(struct task_struct *p)
 {
-	return (p->mm && !(p->flags & PF_BORROWED_MM));
+	return !(p->flags & PF_FREEZER_NOSIG);
 }
 
 /**
  *	freeze_task - send a freeze request to given task
  *	@p: task to send the request to
- *	@with_mm_only: if set, the request will only be sent if the task has its
- *		own mm
- *	Return value: 0, if @with_mm_only is set and the task has no mm of its
- *		own or the task is frozen, 1, otherwise
+ *	@sig_only: if set, the request will only be sent if the task has the
+ *		PF_FREEZER_NOSIG flag unset
+ *	Return value: 'false', if @sig_only is set and the task has
+ *		PF_FREEZER_NOSIG set or the task is frozen, 'true', otherwise
  *
- *	The freeze request is sent by seting the tasks's TIF_FREEZE flag and
+ *	The freeze request is sent by setting the tasks's TIF_FREEZE flag and
  *	either sending a fake signal to it or waking it up, depending on whether
- *	or not it has its own mm (ie. it is a user land task).  If @with_mm_only
- *	is set and the task has no mm of its own (ie. it is a kernel thread),
- *	its TIF_FREEZE flag should not be set.
- *
- *	The task_lock() is necessary to prevent races with exit_mm() or
- *	use_mm()/unuse_mm() from occuring.
+ *	or not it has PF_FREEZER_NOSIG set.  If @sig_only is set and the task
+ *	has PF_FREEZER_NOSIG set (ie. it is a typical kernel thread), its
+ *	TIF_FREEZE flag will not be set.
  */
-static int freeze_task(struct task_struct *p, int with_mm_only)
+static bool freeze_task(struct task_struct *p, bool sig_only)
 {
-	int ret = 1;
+	/*
+	 * We first check if the task is freezing and next if it has already
+	 * been frozen to avoid the race with frozen_process() which first marks
+	 * the task as frozen and next clears its TIF_FREEZE.
+	 */
+	if (!freezing(p)) {
+		rmb();
+		if (frozen(p))
+			return false;
 
-	task_lock(p);
-	if (freezing(p)) {
-		if (has_mm(p)) {
-			if (!signal_pending(p))
-				fake_signal_wake_up(p);
-		} else {
-			if (with_mm_only)
-				ret = 0;
-			else
-				wake_up_state(p, TASK_INTERRUPTIBLE);
-		}
+		if (!sig_only || should_send_signal(p))
+			set_freeze_flag(p);
+		else
+			return false;
+	}
+
+	if (should_send_signal(p)) {
+		if (!signal_pending(p))
+			fake_signal_wake_up(p);
+	} else if (sig_only) {
+		return false;
 	} else {
-		rmb();
-		if (frozen(p)) {
-			ret = 0;
-		} else {
-			if (has_mm(p)) {
-				set_freeze_flag(p);
-				fake_signal_wake_up(p);
-			} else {
-				if (with_mm_only) {
-					ret = 0;
-				} else {
-					set_freeze_flag(p);
-					wake_up_state(p, TASK_INTERRUPTIBLE);
-				}
-			}
-		}
+		wake_up_state(p, TASK_INTERRUPTIBLE);
 	}
-	task_unlock(p);
-	return ret;
+
+	return true;
 }
 
 static void cancel_freezing(struct task_struct *p)
@@ -156,7 +143,7 @@ static void cancel_freezing(struct task_
 	}
 }
 
-static int try_to_freeze_tasks(int freeze_user_space)
+static int try_to_freeze_tasks(bool sig_only)
 {
 	struct task_struct *g, *p;
 	unsigned long end_time;
@@ -175,7 +162,7 @@ static int try_to_freeze_tasks(int freez
 			if (frozen(p) || !freezeable(p))
 				continue;
 
-			if (!freeze_task(p, freeze_user_space))
+			if (!freeze_task(p, sig_only))
 				continue;
 
 			/*
@@ -235,13 +222,13 @@ int freeze_processes(void)
 	int error;
 
 	printk("Freezing user space processes ... ");
-	error = try_to_freeze_tasks(FREEZER_USER_SPACE);
+	error = try_to_freeze_tasks(true);
 	if (error)
 		goto Exit;
 	printk("done.\n");
 
 	printk("Freezing remaining freezable tasks ... ");
-	error = try_to_freeze_tasks(FREEZER_KERNEL_THREADS);
+	error = try_to_freeze_tasks(false);
 	if (error)
 		goto Exit;
 	printk("done.");
@@ -251,7 +238,7 @@ int freeze_processes(void)
 	return error;
 }
 
-static void thaw_tasks(int thaw_user_space)
+static void thaw_tasks(bool nosig_only)
 {
 	struct task_struct *g, *p;
 
@@ -260,7 +247,7 @@ static void thaw_tasks(int thaw_user_spa
 		if (!freezeable(p))
 			continue;
 
-		if (!p->mm == thaw_user_space)
+		if (nosig_only && should_send_signal(p))
 			continue;
 
 		thaw_process(p);
@@ -271,8 +258,8 @@ static void thaw_tasks(int thaw_user_spa
 void thaw_processes(void)
 {
 	printk("Restarting tasks ... ");
-	thaw_tasks(FREEZER_KERNEL_THREADS);
-	thaw_tasks(FREEZER_USER_SPACE);
+	thaw_tasks(true);
+	thaw_tasks(false);
 	schedule();
 	printk("done.\n");
 }
Index: linux-2.6/kernel/kthread.c
===================================================================
--- linux-2.6.orig/kernel/kthread.c
+++ linux-2.6/kernel/kthread.c
@@ -234,7 +234,7 @@ int kthreadd(void *unused)
 	set_user_nice(tsk, KTHREAD_NICE_LEVEL);
 	set_cpus_allowed(tsk, CPU_MASK_ALL);
 
-	current->flags |= PF_NOFREEZE;
+	current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;
 
 	for (;;) {
 		set_current_state(TASK_INTERRUPTIBLE);
--
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