Re: [PATCH] PM/Freezer/Docs: Document the beauty of freeze/thaw semantics
From: Rafael J. Wysocki
Date: Thu Feb 02 2012 - 14:33:16 EST
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@xxxxxxxxxxxxxxxxxx>
> 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:
> + - freezes only userspace tasks
> + - freezes all tasks (including kernel threads) because we can't freeze
> + kernel threads without freezing userspace tasks
> + - 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
> + - 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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