|author||Marcos Paulo de Souza <firstname.lastname@example.org>||2012-04-29 22:29:30 +0200|
|committer||Rafael J. Wysocki <email@example.com>||2012-04-29 22:29:30 +0200|
PM / Freezer / Docs: Update documentation about freezing of tasks
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 <firstname.lastname@example.org> Signed-off-by: Srivatsa S. Bhat <email@example.com> Signed-off-by: Rafael J. Wysocki <firstname.lastname@example.org>
1 files changed, 19 insertions, 18 deletions
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt
index ec715cd78fb..6ec291ea1c7 100644
@@ -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
+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:
@@ -53,7 +54,7 @@ following one:
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.