Re: [PATCH 2/2] sched: Introduce set_special_state()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 01, 2018 at 03:59:24PM +0200, Oleg Nesterov wrote:
> On 05/01, Peter Zijlstra wrote:
> >
> > The only code I found that seems to care is ptrace_attach(), where we
> > wait for JOBCTL_TRAPPING to get cleared. That same function has a
> > comment about hiding the STOPPED -> RUNNING -> TRACED transition. So I'm
> > assuming it needs to observe TRACED if it observes !TRAPPING.
> 
> Yes, exactly.
> 
> > But I don't think there's enough barriers on that end to guarantee this.
> > Any ->state load after wait_on_bit() is, afact, free to have happened
> > before the ->jobctl load.
> 
> do_wait() does set_current_state() before it checks ->state or anything else.

But how are ptrace_attach() and do_wait() related? I guess I'm missing
something fairly fundamental here. Is it userspace doing these two
system calls back-to-back or something?

Is that not a little bit fragile, to rely on a barrier in do_wait()
for this?

Anyway, does the below look ok?

---
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1961,14 +1961,26 @@ static void ptrace_stop(int exit_code, i
 			return;
 	}
 
+	set_special_state(TASK_TRACED);
+
 	/*
 	 * We're committing to trapping.  TRACED should be visible before
 	 * TRAPPING is cleared; otherwise, the tracer might fail do_wait().
 	 * Also, transition to TRACED and updates to ->jobctl should be
 	 * atomic with respect to siglock and should be done after the arch
 	 * hook as siglock is released and regrabbed across it.
+	 *
+	 *     TRACER				    TRACEE
+	 *     ptrace_attach()
+	 * [L]   wait_on_bit(JOBCTL_TRAPPING)	[S] set_special_state(TRACED)
+	 *     do_wait()
+	 *       set_current_state()                smp_wmb();
+	 *       ptrace_do_wait()
+	 *         wait_task_stopped()
+	 *           task_stopped_code()
+	 * [L]         task_is_traced()		[S] task_clear_jobctl_trapping();
 	 */
-	set_special_state(TASK_TRACED);
+	smp_wmb();
 
 	current->last_siginfo = info;
 	current->exit_code = exit_code;