Re: [PATCH] netpoll: Fix carrier detection for drivers that are using phylib

Matt Mackall
2009-07-09T12:54:23Z

On Thu, 2009-07-09 at 02:20 +0400, Anton Vorontsov wrote:
> Using early netconsole and gianfar driver this error pops up:
>
> netconsole: timeout waiting for carrier
>
> It appears that net/core/netpoll.c:netpoll_setup() is using
> cond_resched() in a loop waiting for a carrier.
>
> The thing is that cond_resched() is a no-op when system_state !=
> SYSTEM_RUNNING, and so drivers/net/phy/phy.c's state_queue is never
> scheduled, therefore link detection doesn't work.
>
> I belive that the main problem is in cond_resched()[1], but despite
> how the cond_resched() story ends, it might be a good idea to call
> msleep(1) instead of cond_resched(), as suggested by Andrew Morton.
>
> [1] http://lkml.org/lkml/2009/7/7/463
>
> Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
> ---
>
> On Wed, Jul 08, 2009 at 02:47:44PM -0700, Andrew Morton wrote:
> > (belatedly cc'ing netdev)
> >
> > Original diagnosis:
> >
> > : Using early netconsole and gianfar driver this error pops up:
> > :
> > : netconsole: timeout waiting for carrier
> > :
> > : It appears that net/core/netpoll.c:netpoll_setup() is using
> > : cond_resched() in a loop waiting for a carrier.
> > :
> > : The thing is that cond_resched() is a no-op when system_state !=
> > : SYSTEM_RUNNING, and so drivers/net/phy/phy.c's state_queue is never
> > : scheduled, therefore link detection doesn't work
> >
> > > On Thu, 9 Jul 2009 01:33:31 +0400 Anton Vorontsov <avorontsov@ru.mvista.com> wrote:
> > > On Wed, Jul 08, 2009 at 02:10:24PM -0700, Andrew Morton wrote:
> > > > > On Wed, 8 Jul 2009 09:12:30 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > > > > That said, I do agree that maybe SYSTEM_RUNNING isn't the right check.
> > > > > Testing that the scheduler is initialized may be the more correct one. I
> > > > > think the SYSTEM_RUNNING one just comes from that being used for other
> > > > > debug issues.
> > > >
> > > > Agreed. system_state is too general.
> > > >
> > > > If we specifically want to know whether it is safe to call schedule() then
> > > > let's create a global boolean it_is_safe_to_call_schedule and test that,
> > > > rather than testing something which indirectly and unreliably implies "it
> > > > is safe to call schedule". If that boolean already exists then no-brainer.
> > > >
> > > > All that being said, I wonder if the netconsole code should be using
> > > > msleep(1) instead. Spinning on cond_resched() is a bit rude. But one
> > > > would have to verify that it is safe to call schedule() at this time, and
> > > > for the netconsole caller, this is dubious.
> > >
> > > What do you mean by "verify that it is safe"? If it works,
> > > can I assume that it's safe? ;-) It works, fwiw.
> > >
> >
> > netconsole is supposed to be available as early as possible in boot for
> > obvious reasons. I'd say there's a decent risk now and in the future that
> > netconsole will be initialised prior to the scheduler being available.
> >
> > In fact, if "netconsole: timeout waiting for carrier" newly added to
> > netpoll_setup() a depedency on the scheduler being available then perhaps
> > that was an incorrect change.
>
> 'git blame' says that carrier detection code didn't change since 2.6.12
> (where git history starts), PHYLIB is using workqueue since its
> submission (2.6.13). And SYSTEM_RUNNING check was added in 2.6.16.
> So it's not a new dependency.
>
> The netpoll code is using msleep() just a few lines below cond_resched(),
> so we won't make things worse. ;-)

I think that's an improvement with or without the SYSTEM_RUNNING fix.

Signed-off-by: Matt Mackall <mpm@selenic.com>

> Thanks!
>
> net/core/netpoll.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 9675f31..df30feb 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -740,7 +740,7 @@ int netpoll_setup(struct netpoll *np)
> np->name);
> break;
> }
> - cond_resched();
> + msleep(1);
> }
>
> /* If carrier appears to come up instantly, we don't

--
http://selenic.com : development and support for Mercurial and Linux