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PM / Freezer / Docs: Update documentation about freezing of tasks

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The file Documentation/power/freezing-of-tasks.txt was still referencing
the TIF_FREEZE flag, that was removed by the commit
d88e4cb67197d007fb778d62fe17360e970d5bfa(freezer: remove now unused
TIF_FREEZE).

This patch removes all the references of TIF_FREEZE that were left
behind.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
This commit is contained in:
Marcos Paulo de Souza 2012-04-29 22:29:30 +02:00 committed by Rafael J. Wysocki
parent f8262d4768
commit 26e0f90fde
1 changed files with 19 additions and 18 deletions
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37
Documentation/power/freezing-of-tasks.txt
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@ -9,7 +9,7 @@ architectures).
II. How does it work?
There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN
and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have
PF_NOFREEZE unset (all user space processes and some kernel threads) are
regarded as 'freezable' and treated in a special way before the system enters a
@ -17,30 +17,31 @@ suspend state as well as before a hibernation image is created (in what follows
we only consider hibernation, but the description also applies to suspend).
Namely, as the first step of the hibernation procedure the function
freeze_processes() (defined in kernel/power/process.c) is called. It executes
try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
either wakes them up, if they are kernel threads, or sends fake signals to them,
if they are user space processes. A task that has TIF_FREEZE set, should react
to it by calling the function called __refrigerator() (defined in
kernel/freezer.c), which sets the task's PF_FROZEN flag, changes its state
to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it.
Then, we say that the task is 'frozen' and therefore the set of functions
handling this mechanism is referred to as 'the freezer' (these functions are
defined in kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h).
User space processes are generally frozen before kernel threads.
freeze_processes() (defined in kernel/power/process.c) is called. A system-wide
variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate
whether the system is to undergo a freezing operation. And freeze_processes()
sets this variable. After this, it executes try_to_freeze_tasks() that sends a
fake signal to all user space processes, and wakes up all the kernel threads.
All freezable tasks must react to that by calling try_to_freeze(), which
results in a call to __refrigerator() (defined in kernel/freezer.c), which sets
the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes
it loop until PF_FROZEN is cleared for it. Then, we say that the task is
'frozen' and therefore the set of functions handling this mechanism is referred
to as 'the freezer' (these functions are defined in kernel/power/process.c,
kernel/freezer.c & include/linux/freezer.h). User space processes are generally
frozen before kernel threads.
__refrigerator() must not be called directly. Instead, use the
try_to_freeze() function (defined in include/linux/freezer.h), that checks
the task's TIF_FREEZE flag and makes the task enter __refrigerator() if the
flag is set.
if the task is to be frozen and makes the task enter __refrigerator().
For user space processes try_to_freeze() is called automatically from the
signal-handling code, but the freezable kernel threads need to call it
explicitly in suitable places or use the wait_event_freezable() or
wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
that combine interruptible sleep with checking if TIF_FREEZE is set and calling
try_to_freeze(). The main loop of a freezable kernel thread may look like the
following one:
that combine interruptible sleep with checking if the task is to be frozen and
calling try_to_freeze(). The main loop of a freezable kernel thread may look
like the following one:
set_freezable();
do {
@ -53,7 +54,7 @@ following one:
(from drivers/usb/core/hub.c::hub_thread()).
If a freezable kernel thread fails to call try_to_freeze() after the freezer has
set TIF_FREEZE for it, the freezing of tasks will fail and the entire
initiated a freezing operation, the freezing of tasks will fail and the entire
hibernation operation will be cancelled. For this reason, freezable kernel
threads must call try_to_freeze() somewhere or use one of the
wait_event_freezable() and wait_event_freezable_timeout() macros.