freezer: set PF_SUSPEND_TASK flag on tasks that call freeze_processes

Calling freeze_processes sets a global flag that will cause any
process that calls try_to_freeze to enter the refrigerator.  It
skips sending a signal to the current task, but if the current
task ever hits try_to_freeze, all threads will be frozen and the
system will deadlock.

Set a new flag, PF_SUSPEND_TASK, on the task that calls
freeze_processes.  The flag notifies the freezer that the thread
is involved in suspend and should not be frozen.  Also add a
WARN_ON in thaw_processes if the caller does not have the
PF_SUSPEND_TASK flag set to catch if a different task calls
thaw_processes than the one that called freeze_processes, leaving
a task with PF_SUSPEND_TASK permanently set on it.

Threads that spawn off a task with PF_SUSPEND_TASK set (which
swsusp does) will also have PF_SUSPEND_TASK set, preventing them
from freezing while they are helping with suspend, but they need
to be dead by the time suspend is triggered, otherwise they may
run when userspace is expected to be frozen.  Add a WARN_ON in
thaw_processes if more than one thread has the PF_SUSPEND_TASK
flag set.

Reported-and-tested-by: Michael Leun <lkml20130126@newton.leun.net>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

2 files changed, 12 insertions, 1 deletions

diff --git a/kernel/freezer.c b/kernel/freezer.c
index 8b2afc1c9df0..b462fa197517 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -33,7 +33,7 @@ static DEFINE_SPINLOCK(freezer_lock);
  */
 bool freezing_slow_path(struct task_struct *p)
 {
-        if (p->flags & PF_NOFREEZE)
+        if (p->flags & (PF_NOFREEZE | PF_SUSPEND_TASK))
                 return false;
 
         if (pm_nosig_freezing || cgroup_freezing(p))
diff --git a/kernel/power/process.c b/kernel/power/process.c
index fc0df8486449..06ec8869dbf1 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -109,6 +109,8 @@ static int try_to_freeze_tasks(bool user_only)
 
 /**
  * freeze_processes - Signal user space processes to enter the refrigerator.
+ * The current thread will not be frozen.  The same process that calls
+ * freeze_processes must later call thaw_processes.
  *
  * On success, returns 0.  On failure, -errno and system is fully thawed.
  */
@@ -120,6 +122,9 @@ int freeze_processes(void)
         if (error)
                 return error;
 
+        /* Make sure this task doesn't get frozen */
+        current->flags |= PF_SUSPEND_TASK;
+
         if (!pm_freezing)
                 atomic_inc(&system_freezing_cnt);
 
@@ -168,6 +173,7 @@ int freeze_kernel_threads(void)
 void thaw_processes(void)
 {
         struct task_struct *g, *p;
+        struct task_struct *curr = current;
 
         if (pm_freezing)
                 atomic_dec(&system_freezing_cnt);
@@ -182,10 +188,15 @@ void thaw_processes(void)
 
         read_lock(&tasklist_lock);
         do_each_thread(g, p) {
+                /* No other threads should have PF_SUSPEND_TASK set */
+                WARN_ON((p != curr) && (p->flags & PF_SUSPEND_TASK));
                 __thaw_task(p);
         } while_each_thread(g, p);
         read_unlock(&tasklist_lock);
 
+        WARN_ON(!(curr->flags & PF_SUSPEND_TASK));
+        curr->flags &= ~PF_SUSPEND_TASK;
+
         usermodehelper_enable();
 
         schedule();