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>
diff --git a/kernel/signal.c b/kernel/signal.c
index 0af8868..5ffb5626 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1908,7 +1908,7 @@
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 @@
}
/*
- * 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 @@
}
/* Now we don't run again until woken by SIGCONT or SIGKILL */
- schedule();
+ freezable_schedule();
return true;
} else {
/*
@@ -2200,15 +2193,14 @@
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