linux-kernel - Re: [PATCH] PM/Freezer/Docs: Document the beauty of freeze/thaw semantics

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Date:	Thu, 2 Feb 2012 20:36:58 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tj@...nel.org, pavel@....cz, len.brown@...el.com,
	rdunlap@...otime.net, linux-pm@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PM/Freezer/Docs: Document the beauty of freeze/thaw semantics

On Thursday, February 02, 2012, Srivatsa S. Bhat wrote:
> The way the different freeze/thaw functions encapsulate each other are quite
> lovely from a design point of view. And as a side-effect, the way in which
> they are invoked (cleaning up on failure for example) differs significantly
> from how usual functions are dealt with. This is because of the underlying
> semantics that govern the freezing and thawing of various tasks.
> 
> This subtle aspect that differentiates these functions from the rest, is
> worth documenting.
> 
> Many thanks to Tejun Heo for providing enlightenment on this topic.
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Applied.

Thanks,
Rafael


> ---
> 
>  Documentation/power/freezing-of-tasks.txt |   21 +++++++++++++++++++++
>  1 files changed, 21 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt
> index ebd7490..ec715cd 100644
> --- a/Documentation/power/freezing-of-tasks.txt
> +++ b/Documentation/power/freezing-of-tasks.txt
> @@ -63,6 +63,27 @@ devices have been reinitialized, the function thaw_processes() is called in
>  order to clear the PF_FROZEN flag for each frozen task.  Then, the tasks that
>  have been frozen leave __refrigerator() and continue running.
>  
> +
> +Rationale behind the functions dealing with freezing and thawing of tasks:
> +-------------------------------------------------------------------------
> +
> +freeze_processes():
> +  - freezes only userspace tasks
> +
> +freeze_kernel_threads():
> +  - freezes all tasks (including kernel threads) because we can't freeze
> +    kernel threads without freezing userspace tasks
> +
> +thaw_kernel_threads():
> +  - thaws only kernel threads; this is particularly useful if we need to do
> +    anything special in between thawing of kernel threads and thawing of
> +    userspace tasks, or if we want to postpone the thawing of userspace tasks
> +
> +thaw_processes():
> +  - thaws all tasks (including kernel threads) because we can't thaw userspace
> +    tasks without thawing kernel threads
> +
> +
>  III. Which kernel threads are freezable?
>  
>  Kernel threads are not freezable by default.  However, a kernel thread may clear
> 
> --
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> 

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