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From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
To: Tejun Heo <tj@kernel.org>
Cc: rjw@sisk.pl, pavel@ucw.cz, len.brown@intel.com,
	linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/4] PM/Freezer: Make thaw_processes() thaw only userspace tasks
Date: Tue, 31 Jan 2012 05:39:00 +0530	[thread overview]
Message-ID: <4F27311C.5020200@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120130233055.GE27616@google.com>

On 01/31/2012 05:00 AM, Tejun Heo wrote:

> On Tue, Jan 31, 2012 at 04:44:48AM +0530, Srivatsa S. Bhat wrote:
>> Currently the situation is:
>>
>> freeze_processes() - freezes only userspace tasks
>> freeze_kernel_threads() - freezes only kernel threads
>> thaw_kernel_threads() - thaws only kernel threads
>> thaw_processes() - thaws *everything* (both userspace tasks and kernel threads)
> 
> Umm... I don't really get this.  Why is this a problem?  The list is
> not even correct.  freeze_kernel_threads() doesn't freeze "only"
> kernel threads.  It freezes all threads "including" kernel threads and


Oh, you are are right - the list is incorrect. I guess I got carried away
thinking about thaw_kernel_threads().

> that's only natural because you can't freeze kernel threads without
> freezing userland threads and of course you can't thaw userland
> threads without thawing kernel threads.
>

> The system simply won't work if you do it otherwise and making them

> disjoint operations increases the chance of bugs.  These operations
> are naturally enclosed within each other and trying to break them
> apart isn't a good idea.
> 


Yeah, I get it now.. Thanks for the explanation!

> What's the problem you're trying to solve here?  I don't really see
> code clean up.  Code is different but not necessarily cleaner and FWIW
> it seems more unnatural and brittle to me.
> 


The thing is that, I wanted to avoid a bug in the patch posted at
https://lkml.org/lkml/2012/1/29/47 as explained in the link.

So I guess I should have simply done:

freeze_kernel_threads() calls thaw_kernel_threads() upon error.
The caller of freeze_kernel_threads() will call thaw_processes() if
necessary.

This way even the SNAPSHOT_CREATE_IMAGE ioctl would remain safe.

I'll think it through again and post an updated patch.
Thank you very much for the review!

Regards,
Srivatsa S. Bhat


  reply	other threads:[~2012-01-31  0:09 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-30 23:14 [PATCH v2 0/4] PM / Freezer: Fix the semantics of thaw_processes() and thaw_kernel_threads() Srivatsa S. Bhat
2012-01-30 23:14 ` [PATCH v2 1/4] PM/Freezer: Use thaw_kernel_threads() in preparation for changes to thaw_processes() Srivatsa S. Bhat
2012-01-30 23:14 ` [PATCH v2 2/4] PM/Freezer: Make thaw_processes() thaw only userspace tasks Srivatsa S. Bhat
2012-01-30 23:30   ` Tejun Heo
2012-01-31  0:09     ` Srivatsa S. Bhat [this message]
2012-01-31  0:12       ` Tejun Heo
2012-02-01 14:03       ` Pavel Machek
2012-01-30 23:15 ` [PATCH v2 3/4] PM/Hibernate: Thaw kernel threads in hibernation_snapshot() in error/test path Srivatsa S. Bhat
2012-01-30 23:15 ` [PATCH v2 4/4] PM/Hibernate: Refactor and simplify freezer_test_done Srivatsa S. Bhat

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