Re: [PATCH] PM/Freezer: Thaw only kernel threads if freezing of kernel threads fails

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 02/02/2012 04:06 AM, Tejun Heo wrote:

> On Thu, Feb 02, 2012 at 04:01:12AM +0530, Srivatsa S. Bhat wrote:
>> So, modify freeze_kernel_threads() to thaw only kernel threads in case of
>> freezing failure.
>>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 


Thanks a lot!

> with nits below,
> 
> 
> Heh, the original ?: code was scary.  That said, IMHO, the above seems
> a bit too verbose.  Functions usually shouldn't have side effects on
> failure and the above code is usual in that sense and it would be nice
> to mention the reformatting (from cryptic to sane) in the commit log.


Ok :-)

> Also, how many columns are you flowing the text to?  It seems quite
> narrower than they usually should be.


Oh right, I didn't give attention to that. I'll try to fit it in lesser
number of lines. But, should I really cut short on the explanation itself?
I remember Nigel had requested some comments on a different patch
on roughly similar code path https://lkml.org/lkml/2012/1/28/111.
So I thought it would be a good idea to explicitly document the differences,
to be clear...

Here is the updated patch:

From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Subject: [PATCH] PM/Freezer: Thaw only kernel threads if freezing of kernel threads fails

If freezing of kernel threads fails, we are expected to automatically thaw
tasks in the error recovery path. However, at times, we encounter situations
in which we would like the automatic error recovery path to thaw only the
kernel threads, because we want to be able to do some more cleanup before
we thaw userspace. Something like:

error = freeze_kernel_threads();
if (error) {
	/* Do some cleanup */

	/* Only then thaw userspace tasks*/
	thaw_processes();
}

An example of such a situation is where we freeze/thaw filesystems during
suspend/hibernation. There, if freezing of kernel threads fails, we would
like to thaw the frozen filesystems before thawing the userspace tasks.

So, modify freeze_kernel_threads() to thaw only kernel threads in case of
freezing failure. And change suspend_freeze_processes() accordingly.
(At the same time, let us also get rid of the rather cryptic usage of the
conditional operator (:?) in that function.)

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
---

 kernel/power/power.h   |   24 ++++++++++++++++++++++--
 kernel/power/process.c |    7 +++++--
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/kernel/power/power.h b/kernel/power/power.h
index 0c4defe..21724ee 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -231,8 +231,28 @@ extern int pm_test_level;
 #ifdef CONFIG_SUSPEND_FREEZER
 static inline int suspend_freeze_processes(void)
 {
-	int error = freeze_processes();
-	return error ? : freeze_kernel_threads();
+	int error;
+
+	error = freeze_processes();
+
+	/*
+	 * freeze_processes() automatically thaws every task if freezing
+	 * fails. So we need not do anything extra upon error.
+	 */
+	if (error)
+		goto Finish;
+
+	error = freeze_kernel_threads();
+
+	/*
+	 * freeze_kernel_threads() thaws only kernel threads upon freezing
+	 * failure. So we have to thaw the userspace tasks ourselves.
+	 */
+	if (error)
+		thaw_processes();
+
+ Finish:
+	return error;
 }
 
 static inline void suspend_thaw_processes(void)
diff --git a/kernel/power/process.c b/kernel/power/process.c
index eeca003..7e42645 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -143,7 +143,10 @@ int freeze_processes(void)
 /**
  * freeze_kernel_threads - Make freezable kernel threads go to the refrigerator.
  *
- * On success, returns 0.  On failure, -errno and system is fully thawed.
+ * On success, returns 0.  On failure, -errno and only the kernel threads are
+ * thawed, so as to give a chance to the caller to do additional cleanups
+ * (if any) before thawing the userspace tasks. So, it is the responsibility
+ * of the caller to thaw the userspace tasks, when the time is right.
  */
 int freeze_kernel_threads(void)
 {
@@ -159,7 +162,7 @@ int freeze_kernel_threads(void)
 	BUG_ON(in_atomic());
 
 	if (error)
-		thaw_processes();
+		thaw_kernel_threads();
 	return error;
 }