linux-kernel - Re: [DRIVER SUBMISSION] DRBD wants to go mainline

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Message-ID: <20070724073650.GG3287@kernel.dk>
Date:	Tue, 24 Jul 2007 09:36:51 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	Satyam Sharma <satyam.sharma@...il.com>,
	Kyle Moffett <mrlinuxman@....com>,
	Andrew Morton <akpm@...l.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [DRIVER SUBMISSION] DRBD wants to go mainline

On Mon, Jul 23 2007, Lars Ellenberg wrote:
> On Mon, Jul 23, 2007 at 07:10:58PM +0530, Satyam Sharma wrote:
> > On 7/23/07, Lars Ellenberg <lars.ellenberg@...bit.com> wrote:
> > >On Sun, Jul 22, 2007 at 09:32:02PM -0400, Kyle Moffett wrote:
> > >[...]
> > >> Don't use signals between kernel threads, use proper primitives like
> > >> notifiers and waitqueues, which means you should also probably switch 
> > >away
> > >> from kernel_thread() to the kthread_*() APIs.  Also you should fix this
> > >> FIXME or remove it if it no longer applies:-D.
> > >
> > >right.
> > >but how to I tell a network thread in tcp_recvmsg to stop early,
> > >without using signals?
> > 
> > 
> > Yup, kthreads API cannot handle (properly stop) kernel threads that want
> > to sleep on possibly-blocking-forever-till-signalled-functions such as
> > tcp_recvmsg or skb_recv_datagram etc etc.
> > 
> > There are two workarounds:
> > 1. Use sk_rcvtimeo and related while-continue logic
> > 2. force_sig(SIGKILL) to your kernel thread just before kthread_stop
> >   (note that you don't need to allow / write code to handle / etc signals
> >   in your kthread code -- force_sig will work automatically)
> 
> this is not only at stop time.
> for example our "drbd_asender" thread
> does receive as well as send, and the sending
> latency is crucial to performance, while the recv
> will not timeout for the next few seconds.
> 
> > >> +/* THINK maybe we actually want to use the default "event/%s" worker 
> > >threads
> > >> + * or similar in linux 2.6, which uses per cpu data and threads.
> > >> + *
> > >> + * To be general, this might need a spin_lock member.
> > >> + * For now, please use the mdev->req_lock to protect list_head,
> > >> + * see drbd_queue_work below.
> > >> + */
> > >> +struct drbd_work_queue {
> > >> +       struct list_head q;
> > >> +       struct semaphore s; /* producers up it, worker down()s it */
> > >> +       spinlock_t q_lock;  /* to protect the list. */
> > >> +};
> > >>
> > >> Umm, how about fixing this to actually use proper workqueues or something
> > >> instead of this open-coded mess?
> > >
> > >unlikely to happen "right now".
> > >but it is on our todo list...
> > 
> > It should be easier to do it now (if you defer it for later, the code will
> > only grow more and more complex). Also, removing this gunk from
> > your driver will clearly make it smaller, and easier for us to review :-)
> 
> and will poison the generic work queues with stuff that might block
> somewhere deep in the tcp stack. and where we are not able to cancel it.
> not exactly desirable, either.
> but maybe I am missing something?

Create your own (single threaded) work queue using
create_singlethread_workqueue().

-- 
Jens Axboe

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