freezer: do not send signals to kernel threads

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@sisk.pl>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 74 insertions, 33 deletions

diff --git a/kernel/power/process.c b/kernel/power/process.c
index dba2f3acb4f8..4da125ee533d 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -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 freeze_user_space)
                         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);