Freezer: avoid freezing kernel threads prematurely

Kernel threads should not have TIF_FREEZE set when user space processes are
being frozen, since otherwise some of them might be frozen prematurely.
To prevent this from happening we can (1) make exit_mm() unset TIF_FREEZE
unconditionally just after clearing tsk->mm and (2) make try_to_freeze_tasks()
check if p->mm is different from zero and PF_BORROWED_MM is unset in p->flags
when user space processes are to be frozen.

Namely, when user space processes are being frozen, we only should set
TIF_FREEZE for tasks that have p->mm different from NULL and don't have
PF_BORROWED_MM set in p->flags.  For this reason task_lock() must be used to
prevent try_to_freeze_tasks() from racing with use_mm()/unuse_mm(), in which
p->mm and p->flags.PF_BORROWED_MM are changed under task_lock(p).  Also, we
need to prevent the following scenario from happening:

* daemonize() is called by a task spawned from a user space code path
* freezer checks if the task has p->mm set and the result is positive
* task enters exit_mm() and clears its TIF_FREEZE
* freezer sets TIF_FREEZE for the task
* task calls try_to_freeze() and goes to the refrigerator, which is wrong at
  that point

This requires us to acquire task_lock(p) before p->flags.PF_BORROWED_MM and
p->mm are examined and release it after TIF_FREEZE is set for p (or it turns
out that TIF_FREEZE should not be set).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 41 insertions, 35 deletions

diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 2d38b1a74662..c8e02de737f6 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -25,7 +25,7 @@ static inline int freezing(struct task_struct *p)
 /*
  * Request that a process be frozen
  */
-static inline void freeze(struct task_struct *p)
+static inline void set_freeze_flag(struct task_struct *p)
 {
         set_tsk_thread_flag(p, TIF_FREEZE);
 }
@@ -33,7 +33,7 @@ static inline void freeze(struct task_struct *p)
 /*
  * Sometimes we may need to cancel the previous 'freeze' request
  */
-static inline void do_not_freeze(struct task_struct *p)
+static inline void clear_freeze_flag(struct task_struct *p)
 {
         clear_tsk_thread_flag(p, TIF_FREEZE);
 }
@@ -56,7 +56,7 @@ static inline int thaw_process(struct task_struct *p)
                 wake_up_process(p);
                 return 1;
         }
-        clear_tsk_thread_flag(p, TIF_FREEZE);
+        clear_freeze_flag(p);
         task_unlock(p);
         return 0;
 }
@@ -129,7 +129,8 @@ static inline void set_freezable(void)
 #else
 static inline int frozen(struct task_struct *p) { return 0; }
 static inline int freezing(struct task_struct *p) { return 0; }
-static inline void freeze(struct task_struct *p) { BUG(); }
+static inline void set_freeze_flag(struct task_struct *p) {}
+static inline void clear_freeze_flag(struct task_struct *p) {}
 static inline int thaw_process(struct task_struct *p) { return 1; }
 
 static inline void refrigerator(void) {}
diff --git a/kernel/exit.c b/kernel/exit.c
index e8af8d0c2483..464c2b172f07 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -45,6 +45,7 @@
 #include <linux/resource.h>
 #include <linux/blkdev.h>
 #include <linux/task_io_accounting_ops.h>
+#include <linux/freezer.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -594,6 +595,8 @@ static void exit_mm(struct task_struct * tsk)
         tsk->mm = NULL;
         up_read(&mm->mmap_sem);
         enter_lazy_tlb(mm, current);
+        /* We don't want this task to be frozen prematurely */
+        clear_freeze_flag(tsk);
         task_unlock(tsk);
         mmput(mm);
 }
diff --git a/kernel/power/process.c b/kernel/power/process.c
index b850173e7561..e1bcdedd1464 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -40,7 +40,7 @@ static inline void frozen_process(void)
                 current->flags |= PF_FROZEN;
                 wmb();
         }
-        clear_tsk_thread_flag(current, TIF_FREEZE);
+        clear_freeze_flag(current);
 }
 
 /* Refrigerator is place where frozen processes are stored :-). */
@@ -75,17 +75,16 @@ void refrigerator(void)
         current->state = save;
 }
 
-static inline void freeze_process(struct task_struct *p)
+static void freeze_task(struct task_struct *p)
 {
         unsigned long flags;
 
         if (!freezing(p)) {
                 rmb();
                 if (!frozen(p)) {
+                        set_freeze_flag(p);
                         if (p->state == TASK_STOPPED)
                                 force_sig_specific(SIGSTOP, p);
-
-                        freeze(p);
                         spin_lock_irqsave(&p->sighand->siglock, flags);
                         signal_wake_up(p, p->state == TASK_STOPPED);
                         spin_unlock_irqrestore(&p->sighand->siglock, flags);
@@ -99,18 +98,13 @@ static void cancel_freezing(struct task_struct *p)
 
         if (freezing(p)) {
                 pr_debug("  clean up: %s\n", p->comm);
-                do_not_freeze(p);
+                clear_freeze_flag(p);
                 spin_lock_irqsave(&p->sighand->siglock, flags);
                 recalc_sigpending_and_wake(p);
                 spin_unlock_irqrestore(&p->sighand->siglock, flags);
         }
 }
 
-static inline int is_user_space(struct task_struct *p)
-{
-        return p->mm && !(p->flags & PF_BORROWED_MM);
-}
-
 static unsigned int try_to_freeze_tasks(int freeze_user_space)
 {
         struct task_struct *g, *p;
@@ -122,20 +116,34 @@ static unsigned int try_to_freeze_tasks(int freeze_user_space)
                 todo = 0;
                 read_lock(&tasklist_lock);
                 do_each_thread(g, p) {
-                        if (!freezeable(p))
-                                continue;
-
-                        if (frozen(p))
+                        if (frozen(p) || !freezeable(p))
                                 continue;
 
-                        if (p->state == TASK_TRACED && frozen(p->parent)) {
-                                cancel_freezing(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 (freeze_user_space && !is_user_space(p))
-                                continue;
-
-                        freeze_process(p);
                         if (!freezer_should_skip(p))
                                 todo++;
                 } while_each_thread(g, p);
@@ -152,22 +160,16 @@ static unsigned int try_to_freeze_tasks(int freeze_user_space)
                  * but it cleans up leftover PF_FREEZE requests.
                  */
                 printk("\n");
-                printk(KERN_ERR "Stopping %s timed out after %d seconds "
+                printk(KERN_ERR "Freezing of %s timed out after %d seconds "
                                 "(%d tasks refusing to freeze):\n",
-                                freeze_user_space ? "user space processes" :
-                                        "kernel threads",
+                                freeze_user_space ? "user space " : "tasks ",
                                 TIMEOUT / HZ, todo);
                 show_state();
                 read_lock(&tasklist_lock);
                 do_each_thread(g, p) {
-                        if (freeze_user_space && !is_user_space(p))
-                                continue;
-
                         task_lock(p);
-                        if (freezeable(p) && !frozen(p) &&
-                            !freezer_should_skip(p))
+                        if (freezing(p) && !freezer_should_skip(p))
                                 printk(KERN_ERR " %s\n", p->comm);
-
                         cancel_freezing(p);
                         task_unlock(p);
                 } while_each_thread(g, p);
@@ -211,7 +213,7 @@ static void thaw_tasks(int thaw_user_space)
                 if (!freezeable(p))
                         continue;
 
-                if (is_user_space(p) == !thaw_user_space)
+                if (!p->mm == thaw_user_space)
                         continue;
 
                 thaw_process(p);