Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT

[Peter Zijlstra]

On Thu, Apr 28, 2022 at 10:59:57PM +0200, Oleg Nesterov wrote:
> On 04/28, Peter Zijlstra wrote:
> >
> > Oleg pointed out that the tracee can already be killed such that
> > fatal_signal_pending() is true. In that case signal_wake_up_state()
> > cannot be relied upon to be responsible for the wakeup -- something
> > we're going to want to rely on.
> 
> Peter, I am all confused...
> 
> If this patch is against the current tree, we don't need it.
> 
> If it is on top of JOBCTL_TRACED/DELAY_WAKEKILL changes (yours or Eric's),
> then it can't help - SIGKILL can come right after the tracee drops siglock
> and calls schedule().

But by that time it will already have set TRACED and signal_wake_up()
wil clear it, no?

> Perhaps I missed something, but let me repeat the 3rd time: I'd suggest
> to simply clear JOBCTL_TRACED along with LISTENING/DELAY_WAKEKILL before
> return to close this race.

I think Eric convinced me there was a problem with that, but I'll go
over it all again in the morning, perhaps I'll reach a different
conclusion :-)