Re: [Update][PATCH] PM / Hibernate: Fix s2disk regression relatedto unlock_system_sleep()

[Srivatsa S. Bhat]

On 01/19/2012 03:52 AM, Rafael J. Wysocki wrote:

> On Wednesday, January 18, 2012, Srivatsa S. Bhat wrote:
>> On 01/19/2012 01:16 AM, Srivatsa S. Bhat wrote:
>>
>>> On 01/19/2012 01:00 AM, Tejun Heo wrote:
>>>
>>>> Hello,
>>>>
>>>> On Thu, Jan 19, 2012 at 12:52:32AM +0530, Srivatsa S. Bhat wrote:
>>>>> Somehow I don't think its a hack, based on my perception as described
>>>>> above. But feel free to prove me wrong :-)
>>>>
>>>> Thanks for the explanation.  Yeah, I agree and it's much simpler this
>>>> way, which is nice.  So, in short, because freezing state can't change
>>>> across lock_system_sleep(), there's no reason to check for freezing
>>>> state on unlock and this nicely resolves the freezer problem together.
>>>>
>>>
>>>
>>> Absolutely!
>>>
>>>> The only thing to be careful is, then, we need to set and clear SKIP
>>>> inside pm_mutex.
>>>>
>>>
>>>
>>> Not exactly. We need to set SKIP before grabbing pm_mutex and clear it
>>> inside pm_mutex. The reason is that we decided to set SKIP in the first
>>> place just to avoid the freezer from declaring failure when we are
>>> blocked on pm_mutex. If we move it to *after* mutex_lock(&pm_mutex), that
>>> original intention itself is not satisfied, and we will hit freezing
>>> failures - IOW making the set and clear exercise useless!
>>>
>>> So, something like this should work perfectly:
>>>
>>> lock_system_sleep()
>>> {
>>> 	freezer_do_not_count();
>>> 	mutex_lock(&pm_mutex);
>>> 	current->flags &= ~PF_FREEZER_SKIP;
>>> }
>>>
>>> But in the interest of making the code look a bit symmetric, we can do:
>>>
>>> lock_system_sleep()
>>> {
>>> 	freezer_do_not_count();
>>> 	mutex_lock(&pm_mutex);
>>> }
>>>
>>> unlock_system_sleep()
>>> {
>>> 	current->flags &= ~PF_FREEZER_SKIP;
>>> 	mutex_unlock(&pm_mutex);
>>> }
>>>  
>>
>>
>> So how about this patch, with the comment updated?
>> (and the changelog updated as well to reflect the movement of code)
>>
>> ---
>> From: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>> 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.
>>
>> While at it, make the code more correct and robust by ensuring that the
>> PF_FREEZER_SKIP flag gets cleared with pm_mutex held, to avoid a race with
>> the freezer.
>>
>> Reported-by: Rafael J. Wysocki <rjw@xxxxxxx>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> 
> So, I'd prefer lock_system_sleep() to be open-coded too, to
> avoid situations in which someone changes freezer_do_not_count()
> and freezer_count() leaving unmodified unlock_system_sleep() behind.
>


Hehe, actually, I had left lock_system_sleep() as it is, intentionally, due to
a far-fetched idea to keep the code sane: I felt that people who go through
this code would naturally feel tempted to replace any open-coded stuff (if
possible) with the appropriate function call. And my rationale was that when
they see that lock_system_sleep() has a function call but
unlock_system_sleep() has been open-coded, they would immediately know that
this was not yet another case of "oh I wasn't aware of this function,
hence I open-coded it", but it was rather "I was very much aware of the
available functions, but I chose to open-code it intentionally - so touch it
only if you really know what you are doing" :-)

And I guess, now that job of guarding the code from "obvious code cleanups"
like that would be done by the comment I added as per Tejun's suggestion.
And moreover, your point to open-code in lock_system_sleep() sounds more
compelling than my far-fetched idea :-) So okay, I will open-code it.. 

> Also, have you actually tested this?


Oh I almost forgot to mention: I have not specifically tested this with the
userspace s2disk utility (part of uswsusp/suspend-utils package) as such.
I have only tested it in general (by invoking hibernate by writing to
/sys/power/disk, and testing different levels of pm_test framework etc) to
ensure that this patch doesn't break anything else.
So, any help with testing the userspace utilities would be great!

So Rafael, could you please check if the following patch solves the
regression for you? Thanks a lot!

---
From: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
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.

While at it, make the code more correct and robust by ensuring that the
PF_FREEZER_SKIP flag gets cleared with pm_mutex held, to avoid a race with
the freezer.

Also, to be on the safer side, open-code freezer_do_not_count() as well
(inside lock_system_sleep()), to ensure that any unrelated modification to
freezer[_do_not]_count() does not break things again!

Reported-by: Rafael J. Wysocki <rjw@xxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

 include/linux/suspend.h |   19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/suspend.h b/include/linux/suspend.h
index 95040cc..91784a4 100644
--- a/include/linux/suspend.h
+++ b/include/linux/suspend.h
@@ -357,14 +357,29 @@ extern bool pm_save_wakeup_count(unsigned int count);
 
 static inline void lock_system_sleep(void)
 {
-	freezer_do_not_count();
+	current->flags |= PF_FREEZER_SKIP;
 	mutex_lock(&pm_mutex);
 }
 
 static inline void unlock_system_sleep(void)
 {
+	/*
+	 * Don't use freezer_count() because we don't want the call to
+	 * try_to_freeze() here.
+	 *
+	 * Reason:
+	 * Fundamentally, we just don't need it, because freezing condition
+	 * doesn't come into effect until we release the pm_mutex lock,
+	 * since the freezer always works with pm_mutex held.
+	 *
+	 * More importantly, in the case of hibernation,
+	 * 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 hibernation to lockup.
+	 */
+	current->flags &= ~PF_FREEZER_SKIP;
 	mutex_unlock(&pm_mutex);
-	freezer_count();
 }
 
 #else /* !CONFIG_PM_SLEEP */


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