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Date: Mon, 14 Dec 2009 21:24:54 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Roland McGrath <roland@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       Ananth Mavinakayanahalli <ananth@in.ibm.com>,
       Christoph Hellwig <hch@infradead.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org, utrace-devel@redhat.com
Subject: Re: [RFC,PATCH 14/14] utrace core
Message-ID: <20091214202454.GA16124@redhat.com>
References: <20091124200220.GA5828@redhat.com> <1259697242.1697.1075.camel@laptop> <20091214002533.3052519@magilla.sf.frob.com> <20091214170327.GA7666@redhat.com> <20091214194441.4D6BFE24E@magilla.sf.frob.com>
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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.