Date:	Mon, 25 Jun 2007 20:46:41 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	"Satyam Sharma" <satyam.sharma@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Oleg Nesterov" <oleg@...sign.ru>,
	"Christoph Hellwig" <hch@...radead.org>
Subject: Re: [PATCH] RFC: have tcp_recvmsg() check kthread_should_stop() and
 treat it as if it were signalled

On Tue, 26 Jun 2007 03:39:23 +0530
"Satyam Sharma" <satyam.sharma@...il.com> wrote:

> Hi Jeff,
> 
> [ Trimmed netdev from Cc: list, added Christoph. ]
> 
> On 6/26/07, Jeff Layton <jlayton@...hat.com> wrote:
> > On Tue, 26 Jun 2007 01:11:20 +0530
> > "Satyam Sharma" <satyam.sharma@...il.com> wrote:
> > > [...]
> > > Yes, why not embed a send_sig(SIGKILL) just before the wake_up_process()
> > > in kthread_stop() itself?
> > >
> > > Looking at some happily out-of-date comments in the kthread code, I can
> > > guess that at some point of time (perhaps very early drafts) Rusty actually
> > > *did* implement the whole kthread_stop() functionality using signals, but
> > > I suspect it might've been discarded and the kthread_stop_info approach
> > > used instead to protect from spurious signals from userspace. (?)
> > >
> > > 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)
> > >
> > > If !kthread_should_stop() && signal_pending(current) => spurious signal,
> > > so just flush and discard (in the kthread).
> > > [...]
> > > Why is it wrong for kthreads to let signals through? We can ignore out
> > > all signals we're not interested in, and flush the spurious ones ...
> > > otherwise there really isn't much those kthreads can do that get blocked
> > > in such functions, is there?
> >
> > Yes, after I wrote that I began to question that assumption too. I was
> > pretty much going on a statement by Christoph Hellwig on an earlier
> > patch that I did:
> 
> Ok, I found both the threads / patches you referred to ...
> 
> > -----[snip]------
> > The right way to fix this is to stop sending signals at all and have
> > a kernel-internal way to get out of kernel_recvmsg.  Uses of signals by
> > kernel thread generally are bugs.
> > -----[snip]------
> >
> > Though this makes no sense to me. I don't see any reason why kthreads
> > can't use signals, and hacking support for breaking out of sleeping
> > functions seems redundant.
> 
> Right, signals _are_ the "signalling" mechanism all through kernel code
> already, anything else would clearly be redundant.
> 
> But I've listened / participated in other discussions about kthreads and
> signals and the general feeling is that (somebody correct me if I'm wrong)
> kernel threads are a kernel _implementation detail_ after all, and good
> design must ensure that userspace be unaware of even their existence.
> And I agree with that, but the real ugly uses of signals by kernel threads
> are those cases where we want to export a full signals-based interface to
> our kthread to userspace (such cases do exist in mainline, I think).
> But that's clearly not the case with the usage here.
> 
> > My latest patch for cifsd has it block all signals from userspace
> > and uses force_sig() instead of send_sig() when trying to stop the
> > thread. This seems to work pretty well and  still insulates the thread
> > from userspace signals.
> 
> Thanks, I find this solution much cleaner too, so now we avoid any
> sort of spuriousness getting in from userspace (and pretty much takes
> care of all the checking-if-spurious-and-flushing business I referred to
> earlier).
> 
> But how about making this part of kthreads proper? Functions such as
> skb_recv_datagram etc are pretty standard (and others such would also
> exist or get added in due time) and it is not exactly intuitive for a developer
> to add a force_sig(SIGKILL) before the kthread_stop() to ensure that the
> kthread using such functions does exit properly. [ I can foresee cases in
> the future when such functions are added to kthreads that did not have
> them previously, and suddenly someone reports a regression that the
> kthread stops exiting cleanly. ]
> 
> Satyam

I have no issue with it. I just didn't feel confident enough  to know
whether that might have harmful side effects.  Right offhand I don't
see why adding a force_sig() to kthread_stop() would be an issue. It
seems like if you're wanting to stop the thread anyway then a signal
probably wouldn't hurt anything.

-- 
Jeff Layton <jlayton@...hat.com>
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