linux-kernel - Re: [PATCH 3/4] OOM, PM: OOM killed task shouldn't escape PM suspend

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Date:	Tue, 21 Oct 2014 16:11:59 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 3/4] OOM, PM: OOM killed task shouldn't escape PM suspend

On Tue 21-10-14 15:42:23, Rafael J. Wysocki wrote:
> On Tuesday, October 21, 2014 03:14:45 PM Michal Hocko wrote:
> > On Tue 21-10-14 14:09:27, Rafael J. Wysocki wrote:
> > [...]
> > > > @@ -131,12 +132,40 @@ int freeze_processes(void)
> > > >  
> > > >  	printk("Freezing user space processes ... ");
> > > >  	pm_freezing = true;
> > > > +	oom_kills_saved = oom_kills_count();
> > > >  	error = try_to_freeze_tasks(true);
> > > >  	if (!error) {
> > > > -		printk("done.");
> > > >  		__usermodehelper_set_disable_depth(UMH_DISABLED);
> > > >  		oom_killer_disable();
> > > > +
> > > > +		/*
> > > > +		 * There might have been an OOM kill while we were
> > > > +		 * freezing tasks and the killed task might be still
> > > > +		 * on the way out so we have to double check for race.
> > > > +		 */
> > > > +		if (oom_kills_count() != oom_kills_saved) {
> > > > +			struct task_struct *g, *p;
> > > > +
> > > > +			read_lock(&tasklist_lock);
> > > > +			for_each_process_thread(g, p) {
> > > > +				if (p == current || freezer_should_skip(p) ||
> > > > +				    frozen(p))
> > > > +					continue;
> > > > +				error = -EBUSY;
> > > > +				goto out_loop;
> > > > +			}
> > > > +out_loop:
> > > 
> > > Well, it looks like this will work here too:
> > > 
> > > 			for_each_process_thread(g, p)
> > > 				if (p != current && !frozen(p) &&
> > > 				    !freezer_should_skip(p)) {
> > > 					error = -EBUSY;
> > > 					break;
> > > 				}
> > > 
> > > or I am helplessly misreading the code.
> > 
> > break will not work because for_each_process_thread is a double loop.
> 
> I see.  In that case I'd do:
> 
>                         for_each_process_thread(g, p)
>                                 if (p != current && !frozen(p) &&
>                                     !freezer_should_skip(p)) {
> 
> 					read_unlock(&tasklist_lock);
> 
> 					__usermodehelper_set_disable_depth(UMH_ENABLED);
> 					printk("OOM in progress.");
>                                         error = -EBUSY;
>                                         goto done;
>                                 }
> 
> to avoid adding the new label that looks odd.

OK, incremental diff on top. I will post the complete patch if you are
happier with this change
---
diff --git a/kernel/power/process.c b/kernel/power/process.c
index a397fa161d11..7a37cf3eb1a2 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -108,6 +108,28 @@ static int try_to_freeze_tasks(bool user_only)
 	return todo ? -EBUSY : 0;
 }
 
+/*
+ * Returns true if all freezable tasks (except for current) are frozen already
+ */
+static bool check_frozen_processes(void)
+{
+	struct task_struct *g, *p;
+	bool ret = true;
+
+	read_lock(&tasklist_lock);
+	for_each_process_thread(g, p) {
+		if (p != current && !freezer_should_skip(p) &&
+		    !frozen(p)) {
+			ret = false;
+			goto done;
+		}
+	}
+done:
+	read_unlock(&tasklist_lock);
+
+	return ret;
+}
+
 /**
  * freeze_processes - Signal user space processes to enter the refrigerator.
  * The current thread will not be frozen.  The same process that calls
@@ -143,25 +165,12 @@ int freeze_processes(void)
 		 * freezing tasks and the killed task might be still
 		 * on the way out so we have to double check for race.
 		 */
-		if (oom_kills_count() != oom_kills_saved) {
-			struct task_struct *g, *p;
-
-			read_lock(&tasklist_lock);
-			for_each_process_thread(g, p) {
-				if (p == current || freezer_should_skip(p) ||
-				    frozen(p))
-					continue;
-				error = -EBUSY;
-				goto out_loop;
-			}
-out_loop:
-			read_unlock(&tasklist_lock);
-
-			if (error) {
-				__usermodehelper_set_disable_depth(UMH_ENABLED);
-				printk("OOM in progress.");
-				goto done;
-			}
+		if (oom_kills_count() != oom_kills_saved &&
+				!check_frozen_processes()) {
+			__usermodehelper_set_disable_depth(UMH_ENABLED);
+			printk("OOM in progress.");
+			error = -EBUSY;
+			goto done;
 		}
 		printk("done.");
 	}
-- 
Michal Hocko
SUSE Labs
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