linux-kernel - Re: [Update][PATCH] PM / Hibernate: Fix s2disk regression related to unlock_system

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Date:	Wed, 18 Jan 2012 22:24:36 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, horms@...ge.net.au,
	"pavel@....cz" <pavel@....cz>, Len Brown <lenb@...nel.org>
Subject: Re: [Update][PATCH] PM / Hibernate: Fix s2disk regression related
 to unlock_system_sleep()

Hello Tejun,

On 01/18/2012 09:12 PM, Tejun Heo wrote:

> Hello,
> 
> On Wed, Jan 18, 2012 at 4:59 AM, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> +       /*
>> +        * Don't use freezer_count() because we don't want the
>> +        * call to try_to_freeze() here.
>> +        */
> 
> Please explain "why" here.
>

Ok, how about the following patch?
----
From: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH] PM / Hibernate: Rewrite unlock_system_sleep() to fix s2disk regression

Commit 33e638b, "PM / Sleep: Use the freezer_count() functions in
[un]lock_system_sleep() APIs" introduced an undesirable change in the
behaviour of unlock_system_sleep() since freezer_count() internally calls
try_to_freeze() - which we don't need in unlock_system_sleep().

And commit bcda53f, "PM / Sleep: Replace mutex_[un]lock(&pm_mutex) with
[un]lock_system_sleep()" made these APIs wide-spread. This caused a
regression in suspend-to-disk where snapshot_read() and snapshot_write()
were getting frozen due to the try_to_freeze embedded in
unlock_system_sleep(), since these functions were invoked when the freezing
condition was still in effect.

Fix this by rewriting unlock_system_sleep() by open-coding freezer_count()
and dropping the try_to_freeze() part. Not only will this fix the
regression but this will also ensure that the API only does what it is
intended to do, and nothing more, under the hood.

Reported-by: Rafael J. Wysocki <rjw@...k.pl>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 include/linux/suspend.h |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/include/linux/suspend.h b/include/linux/suspend.h
index 95040cc..6e76380 100644
--- a/include/linux/suspend.h
+++ b/include/linux/suspend.h
@@ -364,7 +364,18 @@ static inline void lock_system_sleep(void)
 static inline void unlock_system_sleep(void)
 {
 	mutex_unlock(&pm_mutex);
-	freezer_count();
+
+	/*
+	 * Don't use freezer_count() because we don't want the call to
+	 * try_to_freeze() here.
+	 *
+	 * Reason:
+	 * unlock_system_sleep() gets called in snapshot_read() and
+	 * snapshot_write() when the freezing condition is still in effect.
+	 * Which means, if we use try_to_freeze() here, it would make them
+	 * enter the refrigerator, thus causing suspend-to-disk to lockup.
+	 */
+	current->flags &= ~PF_FREEZER_SKIP;
 }

 #else /* !CONFIG_PM_SLEEP */

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