netdev - Re: [PATCH/RFC] sched: Remove SYSTEM_RUNNING checks from cond

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Message-Id: <20090708144744.5555b88d.akpm@linux-foundation.org>
Date:	Wed, 8 Jul 2009 14:47:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	avorontsov@...mvista.com
Cc:	torvalds@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	oleg@...hat.com, mingo@...e.hu, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH/RFC] sched: Remove SYSTEM_RUNNING checks from
 cond_resched*()

(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@...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@...ux-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.

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