Messages in this thread | | | Date | Tue, 1 May 2018 17:10:50 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] sched: Introduce set_special_state() |
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On 05/01, Peter Zijlstra wrote: > > 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?
Yes.
> I guess I'm missing > something fairly fundamental here.
You are missing the fact that ptrace API is very old and ugly ;)
Just one example. If the debugger knows that the task is STOPPED then it has all rights to do, say,
ptrace(PTRACE_ATTACH, pid); BUG_ON(pid != waitpid(pid, WNOHANG));
Or even do another ptrace() request right after ptrace(PTRACE_ATTACH) returns, without do_wait().
And unless my memory fools me, gdb even has some test-cases for this... Not sure, but it certainly looks at tracee->state in /proc before it does PTRACE_ATTACH, because if it was already STOPPED then gdb won't have any notification from the tracee.
> Anyway, does the below look ok?
Yes, thanks.
Oleg.
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