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Date: Wed, 18 Jul 2007 09:39:10 +0200
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Nigel Cunningham <nigel@...el.suspend2.net>,
Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>
Subject: [PATCH -mm 5/5] Freezer: Do not send signals to kernel threads
From: Rafael J. Wysocki <rjw@...k.pl>
The freezer should not send signals to kernel threads, since that may lead to
subtle problems. In particular, commit b74d0deb968e1f85942f17080eace015ce3c332c
has changed recalc_sigpending_tsk() so that it doesn't clear TIF_SIGPENDING.
For this reason, if the freezer continues to send fake signals to kernel threads
and the freezing of kernel threads fails, some of them may be running with
TIF_SIGPENDING set forever.
Accordingly, recalc_sigpending_tsk() shouldn't set the task's TIF_SIGPENDING
flag if TIF_FREEZE is set.
Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
---
Documentation/power/freezing-of-tasks.txt | 31 +++++---
kernel/power/process.c | 107 ++++++++++++++++++++----------
kernel/signal.c | 1
3 files changed, 93 insertions(+), 46 deletions(-)
Index: linux-2.6.22-rc6-mm1/kernel/power/process.c
===================================================================
--- linux-2.6.22-rc6-mm1.orig/kernel/power/process.c 2007-07-11 20:50:28.000000000 +0200
+++ linux-2.6.22-rc6-mm1/kernel/power/process.c 2007-07-11 20:51:48.000000000 +0200
@@ -75,21 +75,79 @@ void refrigerator(void)
__set_current_state(save);
}
-static void freeze_task(struct task_struct *p)
+static void fake_signal_wake_up(struct task_struct *p, int resume)
{
unsigned long flags;
- if (!freezing(p)) {
+ spin_lock_irqsave(&p->sighand->siglock, flags);
+ signal_wake_up(p, resume);
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
+}
+
+static void send_fake_signal(struct task_struct *p)
+{
+ if (p->state == TASK_STOPPED)
+ force_sig_specific(SIGSTOP, p);
+ fake_signal_wake_up(p, p->state == TASK_STOPPED);
+}
+
+static int has_mm(struct task_struct *p)
+{
+ return (p->mm && !(p->flags & PF_BORROWED_MM));
+}
+
+/**
+ * freeze_task - send a freeze request to given task
+ * @p: task to send the request to
+ * @with_mm_only: if set, the request will only be sent if the task has its
+ * own mm
+ * Return value: 0, if @with_mm_only is set and the task has no mm of its
+ * own or the task is frozen, 1, otherwise
+ *
+ * The freeze request is sent by seting the tasks's TIF_FREEZE flag and
+ * either sending a fake signal to it or waking it up, depending on whether
+ * or not it has its own mm (ie. it is a user land task). If @with_mm_only
+ * is set and the task has no mm of its own (ie. it is a kernel thread),
+ * its TIF_FREEZE flag should not be set.
+ *
+ * The task_lock() is necessary to prevent races with exit_mm() or
+ * use_mm()/unuse_mm() from occuring.
+ */
+static int freeze_task(struct task_struct *p, int with_mm_only)
+{
+ int ret = 1;
+
+ task_lock(p);
+ if (freezing(p)) {
+ if (has_mm(p)) {
+ if (!signal_pending(p))
+ fake_signal_wake_up(p, 0);
+ } else {
+ if (with_mm_only)
+ ret = 0;
+ else
+ wake_up_state(p, TASK_INTERRUPTIBLE);
+ }
+ } else {
rmb();
- if (!frozen(p)) {
- set_freeze_flag(p);
- if (p->state == TASK_STOPPED)
- force_sig_specific(SIGSTOP, p);
- spin_lock_irqsave(&p->sighand->siglock, flags);
- signal_wake_up(p, p->state == TASK_STOPPED);
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ if (frozen(p)) {
+ ret = 0;
+ } else {
+ if (has_mm(p)) {
+ set_freeze_flag(p);
+ send_fake_signal(p);
+ } else {
+ if (with_mm_only) {
+ ret = 0;
+ } else {
+ set_freeze_flag(p);
+ wake_up_state(p, TASK_INTERRUPTIBLE);
+ }
+ }
}
}
+ task_unlock(p);
+ return ret;
}
static void cancel_freezing(struct task_struct *p)
@@ -119,31 +177,14 @@ static int try_to_freeze_tasks(int freez
if (frozen(p) || !freezeable(p))
continue;
- if (freeze_user_space) {
- if (p->state == TASK_TRACED &&
- frozen(p->parent)) {
- cancel_freezing(p);
- continue;
- }
- /*
- * Kernel threads should not have TIF_FREEZE set
- * at this point, so we must ensure that either
- * p->mm is not NULL *and* PF_BORROWED_MM is
- * unset, or TIF_FRREZE is left unset.
- * The task_lock() is necessary to prevent races
- * with exit_mm() or use_mm()/unuse_mm() from
- * occuring.
- */
- task_lock(p);
- if (!p->mm || (p->flags & PF_BORROWED_MM)) {
- task_unlock(p);
- continue;
- }
- freeze_task(p);
- task_unlock(p);
- } else {
- freeze_task(p);
+ if (p->state == TASK_TRACED && frozen(p->parent)) {
+ cancel_freezing(p);
+ continue;
}
+
+ if (!freeze_task(p, freeze_user_space))
+ continue;
+
if (!freezer_should_skip(p))
todo++;
} while_each_thread(g, p);
Index: linux-2.6.22-rc6-mm1/kernel/signal.c
===================================================================
--- linux-2.6.22-rc6-mm1.orig/kernel/signal.c 2007-07-11 20:48:04.000000000 +0200
+++ linux-2.6.22-rc6-mm1/kernel/signal.c 2007-07-11 20:51:48.000000000 +0200
@@ -99,7 +99,6 @@ static inline int has_pending_signals(si
static int recalc_sigpending_tsk(struct task_struct *t)
{
if (t->signal->group_stop_count > 0 ||
- (freezing(t)) ||
PENDING(&t->pending, &t->blocked) ||
PENDING(&t->signal->shared_pending, &t->blocked)) {
set_tsk_thread_flag(t, TIF_SIGPENDING);
Index: linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt
===================================================================
--- linux-2.6.22-rc6-mm1.orig/Documentation/power/freezing-of-tasks.txt 2007-07-11 20:50:24.000000000 +0200
+++ linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt 2007-07-11 20:51:48.000000000 +0200
@@ -19,12 +19,13 @@ we only consider hibernation, but the de
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
-sends a fake signal to each of them. A task that receives such a signal and has
-TIF_FREEZE set, should react to it by calling the refrigerator() function
-(defined in kernel/power/process.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 called 'the freezer' (these functions are
+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/power/process.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 and include/linux/freezer.h). User space
processes are generally frozen before kernel threads.
@@ -35,21 +36,27 @@ task enter refrigerator() if the flag is
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. The code to do this may look like the following:
+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:
+ set_freezable();
do {
hub_events();
- wait_event_interruptible(khubd_wait,
- !list_empty(&hub_event_list));
- try_to_freeze();
- } while (!signal_pending(current));
+ wait_event_freezable(khubd_wait,
+ !list_empty(&hub_event_list) ||
+ kthread_should_stop());
+ } while (!kthread_should_stop() || !list_empty(&hub_event_list));
(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
hibernation operation will be cancelled. For this reason, freezable kernel
-threads must call try_to_freeze() somewhere.
+threads must call try_to_freeze() somewhere or use one of the
+wait_event_freezable() and wait_event_freezable_timeout() macros.
After the system memory state has been restored from a hibernation image and
devices have been reinitialized, the function thaw_processes() is called in
-
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