Re: [PATCH -mm 1/2]: PM: Fix handling of stopped tasks

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

On Saturday, 9 December 2006 16:35, Rafael J. Wysocki wrote:
> On Saturday, 9 December 2006 00:36, Pavel Machek wrote:
> > Hi!
> > 
> > > > > I wonder if we should start a test suite ;-).
> > > > > 
> > > > > > This means, however, that with this patch the behavior of a process (gdb)
> > > > > > after the resume may be different to its normal behavior, which is wrong.
> > > > > 
> > > > > Yep.
> > > 
> > > Okay, I think I know what to do so that it works.  The above symptoms are not
> > > present with the appended patch.
> > 
> > Looks good to me. Thanks for you work!
> 
> Well, there's still a race possible in there, if SIGCONT comes after we have
> forced the SIGSTOP and before we call signal_wake_up().
> 
> I think we should force the SIGSTOP and call signal_wake_up() without
> releasing the lock.  I'll try to do something along these lines later today.

I think something like the appended patch will do.

Greetings,
Rafael


Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
 include/linux/sched.h  |    1 +
 kernel/power/process.c |   15 ++++++++-------
 kernel/signal.c        |   16 +++++++++++++++-
 3 files changed, 24 insertions(+), 8 deletions(-)

Index: linux-2.6.19-rc6-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/kernel/power/process.c	2006-12-08 23:20:35.000000000 +0100
+++ linux-2.6.19-rc6-mm2/kernel/power/process.c	2006-12-10 00:37:24.000000000 +0100
@@ -28,8 +28,7 @@ static inline int freezeable(struct task
 	if ((p == current) || 
 	    (p->flags & PF_NOFREEZE) ||
 	    (p->exit_state == EXIT_ZOMBIE) ||
-	    (p->exit_state == EXIT_DEAD) ||
-	    (p->state == TASK_STOPPED))
+	    (p->exit_state == EXIT_DEAD))
 		return 0;
 	return 1;
 }
@@ -61,9 +60,13 @@ static inline void freeze_process(struct
 	unsigned long flags;
 
 	if (!freezing(p)) {
-		freeze(p);
 		spin_lock_irqsave(&p->sighand->siglock, flags);
-		signal_wake_up(p, 0);
+		freeze(p);
+		if (p->state == TASK_STOPPED) {
+			force_sigstop_and_wake_up(p);
+		} else {
+			signal_wake_up(p, 0);
+		}
 		spin_unlock_irqrestore(&p->sighand->siglock, flags);
 	}
 }
@@ -103,9 +106,7 @@ static unsigned int try_to_freeze_tasks(
 			if (frozen(p))
 				continue;
 
-			if (p->state == TASK_TRACED &&
-			    (frozen(p->parent) ||
-			     p->parent->state == TASK_STOPPED)) {
+			if (p->state == TASK_TRACED && frozen(p->parent)) {
 				cancel_freezing(p);
 				continue;
 			}
Index: linux-2.6.19-rc6-mm2/kernel/signal.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/kernel/signal.c	2006-12-08 23:20:35.000000000 +0100
+++ linux-2.6.19-rc6-mm2/kernel/signal.c	2006-12-10 00:43:39.000000000 +0100
@@ -962,6 +962,18 @@ force_sig_specific(int sig, struct task_
 }
 
 /*
+ * Force a SIGSTOP and make the task wake up.
+ *
+ * This is needed for the freezing of stopped tasks, because we want them to
+ * enter the refrigerator and be stopped again immediately after leaving it.
+ */
+void force_sigstop_and_wake_up(struct task_struct *t)
+{
+	specific_send_sig_info(SIGSTOP, SEND_SIG_FORCED, t);
+	signal_wake_up(t, 1);
+}
+
+/*
  * Test if P wants to take SIG.  After we've checked all threads with this,
  * it's equivalent to finding no threads not blocking SIG.  Any threads not
  * blocking SIG were ruled out because they are not running and already
@@ -1829,7 +1841,9 @@ finish_stop(int stop_count)
 		read_unlock(&tasklist_lock);
 	}
 
-	schedule();
+	do {
+		schedule();
+	} while (try_to_freeze());
 	/*
 	 * Now we don't run again until continued.
 	 */
Index: linux-2.6.19-rc6-mm2/include/linux/sched.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/linux/sched.h	2006-12-10 00:20:07.000000000 +0100
+++ linux-2.6.19-rc6-mm2/include/linux/sched.h	2006-12-10 00:42:44.000000000 +0100
@@ -1350,6 +1350,7 @@ extern int kill_pg_info(int, struct sigi
 extern void do_notify_parent(struct task_struct *, int);
 extern void force_sig(int, struct task_struct *);
 extern void force_sig_specific(int, struct task_struct *);
+extern void force_sigstop_and_wake_up(struct task_struct *t);
 extern int send_sig(int, struct task_struct *, int);
 extern void zap_other_threads(struct task_struct *p);
 extern int kill_pg(pid_t, int, int);

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