netdev - Re: [PATCH] RFC: have tcp_recvmsg() check kthread_should_stop() and treat it as if it were signalled

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Message-ID: <a781481a0706261829l4a5c19cey7e1e75157fb74ce2@mail.gmail.com>
Date:	Wed, 27 Jun 2007 06:59:35 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Oleg Nesterov" <oleg@...sign.ru>
Cc:	"Jeff Layton" <jlayton@...hat.com>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Herbert Xu" <herbert@...dor.apana.org.au>, netdev@...r.kernel.org,
	linux-cifs-client@...ts.samba.org
Subject: Re: [PATCH] RFC: have tcp_recvmsg() check kthread_should_stop() and treat it as if it were signalled

On 6/27/07, Satyam Sharma <satyam.sharma@...il.com> wrote:
> [...]
> On 6/26/07, Oleg Nesterov <oleg@...sign.ru> wrote:
> > On 06/26, Satyam Sharma wrote:
> [...]
> > > So could we have signals in _addition_ to kthread_stop_info and change
> > > kthread_should_stop() to check for both:
> > >
> > > kthread_stop_info.k == current && signal_pending(current)
> >
> > No, this can't work in general. Some kthreads do flush_signals/dequeue_signal,
> > so TIF_SIGPENDING can be lost anyway.
>
> Yup, I had thought of precisely this issue yesterday as well. The mental note
> I made to myself was that the force_sig(SIGKILL) and wake_up_process() in
> kthread_stop() must be atomic so that the following race is not possible:

Hmm, the issue seems to have more to do with the ordering of
flush_signals() w.r.t. checking kthread_should_stop() in the kthread's
code. I thought about how to tackle this, but there's no easy way to make
the stuff atomic like I thought earlier. The problem, like you mentioned,
is if the target kthread proactively flushes its signals by hand *before*
checking kthread_should_stop().

The only way out seems to be to simply outlaw flush_signals() in kthreads
(or anything to do with signals), but that would be impossible to enforce ...

Satyam
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