[PATCH] signal: break out of wait loops on kthread_stop()

["Jason A. Donenfeld" <Jason@zx2c4.com>]

I was recently surprised to learn that msleep_interruptible(),
wait_for_completion_interruptible_timeout(), and related functions
simply hung when I called kthread_stop() on kthreads using them. The
solution to fixing the case with msleep_interruptible() was more simply
to move to schedule_timeout_interruptible(). Why?

The reason is that msleep_interruptible(), and many functions just like
it, has a loop like this:

        while (timeout && !signal_pending(current))
                timeout = schedule_timeout_interruptible(timeout);

The call to kthread_stop() woke up the thread, so schedule_timeout_
interruptible() returned early, but because signal_pending() returned
true, it went back into another timeout, which was never woken up.

This wait loop pattern is common to various pieces of code, and I
suspect that subtle misuse in a kthread that caused a deadlock in the
code I looked at last week is also found elsewhere.

So this commit causes signal_pending() to return true when
kthread_stop() is called. This is already what's done for
TIF_NOTIFY_SIGNAL, for these same purposes of breaking out of wait
loops, so a similar KTHREAD_SHOULD_STOP check isn't too much different.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 include/linux/kthread.h      | 1 +
 include/linux/sched/signal.h | 9 +++++++++
 kernel/kthread.c             | 6 ++++++
 3 files changed, 16 insertions(+)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 30e5bec81d2b..7061dde23237 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -87,6 +87,7 @@ void kthread_bind(struct task_struct *k, unsigned int cpu);
 void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask);
 int kthread_stop(struct task_struct *k);
 bool kthread_should_stop(void);
+bool __kthread_should_stop(struct task_struct *k);
 bool kthread_should_park(void);
 bool __kthread_should_park(struct task_struct *k);
 bool kthread_freezable_should_stop(bool *was_frozen);
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index cafbe03eed01..08700c65b806 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -11,6 +11,7 @@
 #include <linux/refcount.h>
 #include <linux/posix-timers.h>
 #include <linux/mm_types.h>
+#include <linux/kthread.h>
 #include <asm/ptrace.h>
 
 /*
@@ -397,6 +398,14 @@ static inline int signal_pending(struct task_struct *p)
 	 */
 	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
 		return 1;
+
+	/*
+	 * Likewise, KTHREAD_SHOULD_STOP isn't really a signal, but it also
+	 * requires the same behavior, lest wait loops go forever.
+	 */
+	if (unlikely(__kthread_should_stop(p)))
+		return 1;
+
 	return task_sigpending(p);
 }
 
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 3c677918d8f2..7e0743330cd4 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -145,6 +145,12 @@ void free_kthread_struct(struct task_struct *k)
 	kfree(kthread);
 }
 
+bool __kthread_should_stop(struct task_struct *k)
+{
+	return (k->flags & PF_KTHREAD) &&
+	       test_bit(KTHREAD_SHOULD_STOP, &to_kthread(k)->flags);
+}
+
 /**
  * kthread_should_stop - should this kthread return now?
  *
-- 
2.35.1