freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()

try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
to ensure that a task doing STOPPED/TRACED -> RUNNING transition
can't escape freezing. This mostly works, but ptrace_stop() does
not necessarily call schedule(), it can change task->state back to
RUNNING and check freezing() without any lock/barrier in between.

We could add the necessary barrier, but this patch changes
ptrace_stop() and do_signal_stop() to use freezable_schedule().
This fixes the race, freezer_count() and freezer_should_skip()
carefully avoid the race.

And this simplifies the code, try_to_freeze_tasks/update_if_frozen
no longer need to use task_is_stopped_or_traced() checks with the
non trivial assumptions. We can rely on the mechanism which was
specially designed to mark the sleeping task as "frozen enough".

v2: As Tejun pointed out, we can also change get_signal_to_deliver()
and move try_to_freeze() up before 'relock' label.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 6 insertions, 14 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 0af8868525d6..5ffb5626e072 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1908,7 +1908,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
                 preempt_disable();
                 read_unlock(&tasklist_lock);
                 preempt_enable_no_resched();
-                schedule();
+                freezable_schedule();
         } else {
                 /*
                  * By the time we got the lock, our tracer went away.
@@ -1930,13 +1930,6 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
         }
 
         /*
-         * While in TASK_TRACED, we were considered "frozen enough".
-         * Now that we woke up, it's crucial if we're supposed to be
-         * frozen that we freeze now before running anything substantial.
-         */
-        try_to_freeze();
-
-        /*
          * We are back.  Now reacquire the siglock before touching
          * last_siginfo, so that we are sure to have synchronized with
          * any signal-sending on another CPU that wants to examine it.
@@ -2092,7 +2085,7 @@ static bool do_signal_stop(int signr)
                 }
 
                 /* Now we don't run again until woken by SIGCONT or SIGKILL */
-                schedule();
+                freezable_schedule();
                 return true;
         } else {
                 /*
@@ -2200,15 +2193,14 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
         if (unlikely(uprobe_deny_signal()))
                 return 0;
 
-relock:
         /*
-         * We'll jump back here after any time we were stopped in TASK_STOPPED.
-         * While in TASK_STOPPED, we were considered "frozen enough".
-         * Now that we woke up, it's crucial if we're supposed to be
-         * frozen that we freeze now before running anything substantial.
+         * Do this once, we can't return to user-mode if freezing() == T.
+         * do_signal_stop() and ptrace_stop() do freezable_schedule() and
+         * thus do not need another check after return.
          */
         try_to_freeze();
 
+relock:
         spin_lock_irq(&sighand->siglock);
         /*
          * Every stopped thread goes here after wakeup. Check to see if