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From: Denys Vlasenko <vda.linux@googlemail.com>
To: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 11/11] ptrace: implement group stop notification for ptracer
Date: Mon, 9 May 2011 00:42:55 +0200
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Cc: oleg@redhat.com, jan.kratochvil@redhat.com, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org, indan@nul.nu
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On Sunday 08 May 2011 17:49, Tejun Heo wrote:
> Currently there's no way for ptracer to find out whether group stop
> that tracee was in finished other than polling with PTRACE_GETSIGINFO.
> Also, tracer can't detect new group stop started by an untraced thread
> if tracee is already trapped.  This patch implements group stop
> notification for ptracer using INTERRUPT traps.
> 
> When group stop state of a seized tracee changes, JOBCTL_TRAP_NOTIFY
> is set, which triggers INTERRUPT trap but is sticky until the next
> PTRACE_GETSIGINFO.  As GETSIGINFO exports the current group stop
> state, this guarantees that tracer checks the current group stop state
> at least once after group stop state change.  Stickiness is necessary
> because notification trap may race with PTRACE_CONT for other traps
> and get lost.

To clarify on "sticky":

Does this mean that if tracer userspace won't query PTRACE_GETSIGINFO,
but will simply do:

                  waitid(P_PID, tracee, NULL, WSTOPPED);
                  ptrace(PTRACE_CONT, tracee, NULL, NULL);

then kernel will generate INTERRUPT trap again and again
(without letting tracee run) if job stop state of the tracee has changed?


-- 
vda
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