linux-kernel - Re: [PATCH] nbd: Move socket shutdown out of spinlock

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Message-ID: <81303581.gVSuzjqYUx@adelgunde>
Date:	Thu, 12 May 2016 14:43:17 +0200
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Pranay Srivastava <pranjas@...il.com>
Cc:	nbd-general@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] nbd: Move socket shutdown out of spinlock

Hi,

On Thursday 12 May 2016 16:42:31 Pranay Srivastava wrote:
> Hi Markus,
> 
> 
> On Thu, May 12, 2016 at 3:13 PM, Markus Pargmann <mpa@...gutronix.de> wrote:
> > spinlocked ranges should be small and not contain calls into huge
> > subfunctions. Fix my mistake and just get the pointer to the socket
> > instead of doing everything with spinlock held.
> >
> > Reported-by: Mikulas Patocka <mikulas@...bright.com>
> > Signed-off-by: Markus Pargmann <mpa@...gutronix.de>
> > ---
> >  drivers/block/nbd.c | 18 ++++++++++--------
> >  1 file changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > index 0b892eed06a0..157bf3da876e 100644
> > --- a/drivers/block/nbd.c
> > +++ b/drivers/block/nbd.c
> > @@ -173,20 +173,22 @@ static void nbd_end_request(struct nbd_device *nbd, struct request *req)
> >   */
> >  static void sock_shutdown(struct nbd_device *nbd)
> >  {
> > +       struct socket *sock;
> > +
> >         spin_lock_irq(&nbd->sock_lock);
> > +       sock = nbd->sock;
> > +       nbd->sock = NULL;
> > +       spin_unlock_irq(&nbd->sock_lock);
> >
> > -       if (!nbd->sock) {
> > -               spin_unlock_irq(&nbd->sock_lock);
> > +       if (!sock)
> >                 return;
> > -       }
> > +
> > +       del_timer(&nbd->timeout_timer);
> >
> >         dev_warn(disk_to_dev(nbd->disk), "shutting down socket\n");
> > -       kernel_sock_shutdown(nbd->sock, SHUT_RDWR);
> > -       sockfd_put(nbd->sock);
> > -       nbd->sock = NULL;
> > -       spin_unlock_irq(&nbd->sock_lock);
> > +       kernel_sock_shutdown(sock, SHUT_RDWR);
> > +       sockfd_put(sock);
> >
> > -       del_timer(&nbd->timeout_timer);
> >  }
> >
> >  static void nbd_xmit_timeout(unsigned long arg)
> 
> I was concerned about nbd_xmit_timeout as well. There's also a call to
> kernel_sock_shutdown,
> while holding the spin_lock in the timeout. The above is ok for
> sock_shutdown but some kind of change
> is also required in nbd_xmit_timeout as well. My patch addressed both these.

Oh I see thanks. Seems there is some duplicate code in
nbd_xmit_timeout and sock_shutdown. I think nbd_xmit_timeout could
perhaps be simplified?

	static void nbd_xmit_timeout(unsigned long arg)
	{
		struct nbd_device *nbd = (struct nbd_device *)arg;

		if (list_empty(&nbd->queue_head))
			return;

		nbd->timedout = true;
		dev_err(nbd_to_dev(nbd), "Connection timed out, shutting down connection\n");

		sock_shutdown(nbd);
	}

Thanks,

Markus

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