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From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, tglx@linutronix.de,
	linux-kernel@vger.kernel.org, will.deacon@arm.com,
	mpe@ellerman.id.au, bigeasy@linutronix.de, gkohli@codeaurora.org,
	neeraju@codeaurora.org
Subject: Re: [PATCH 2/2] sched: Introduce set_special_state()
Date: Tue, 1 May 2018 17:10:50 +0200	[thread overview]
Message-ID: <20180501151050.GA13094@redhat.com> (raw)
In-Reply-To: <20180501142251.GH12217@hirez.programming.kicks-ass.net>

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.

      reply	other threads:[~2018-05-01 15:10 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-30 14:17 [PATCH 0/2] sched,kthread: Fix TASK_PARKED and special sleep states Peter Zijlstra
2018-04-30 14:17 ` [PATCH 1/2] kthread: Fix kthread_parkme() wait-loop Peter Zijlstra
2018-04-30 14:17 ` [PATCH 2/2] sched: Introduce set_special_state() Peter Zijlstra
2018-04-30 16:45   ` Oleg Nesterov
2018-04-30 19:40     ` Peter Zijlstra
2018-05-01  9:50       ` Peter Zijlstra
2018-05-01 13:59         ` Oleg Nesterov
2018-05-01 14:22           ` Peter Zijlstra
2018-05-01 15:10             ` Oleg Nesterov [this message]

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