Messages in this thread Patch in this message | | | Date | Tue, 1 May 2018 16:22:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] sched: Introduce set_special_state() |
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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;
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