linux-kernel - Re: [RFC,PATCH 14/14] utrace core

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Message-ID: <20091214202454.GA16124@redhat.com>
Date:	Mon, 14 Dec 2009 21:24:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ananth Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, utrace-devel@...hat.com
Subject: Re: [RFC,PATCH 14/14] utrace core

On 12/14, Roland McGrath wrote:
>
> > > In the stopped cases, there are lots of locks and barriers and things
> > > after resuming.  (Oleg?)
> >
> > Every time the tracee resumes after TASK_TRACED it uses utrace->lock
> > to synchronize with utrace_control/etc, it must see any changes.
>
> And TASK_STOPPED?

SIGCONT can wake up the TASK_STOPPED tracee. I don't think the tracer
should ever rely on TASK_STOPPED (utrace never does). If the tracer
needs the "really stopped" tracee we have UTRACE_STOP, and this means
TASK_TRACED.

But I am not sure I understand this part of discussion... In any case
the tracee should see any changes which were done before the wakeup.
But TASK_STOPPED can't guarantee the tracee won't be resumed until
the tracer wakes it up. Of course, TASK_TRACED can't prevent SIGKILL,
but in this case we should only care about "the tracee can't resume
until we drop utrace->lock" case.

Oleg.

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