netdev - Re: [PATCH] netconsole: Disable softirqs in write

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Message-Id: <20081119.014222.193698982.davem@davemloft.net>
Date:	Wed, 19 Nov 2008 01:42:22 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	mingo@...e.hu
Cc:	jarkao2@...il.com, jeff@...zik.org, johannes@...solutions.net,
	wferi@...f.hu, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netconsole: Disable softirqs in write_msg()

From: Ingo Molnar <mingo@...e.hu>
Date: Wed, 19 Nov 2008 10:30:04 +0100

> * Jarek Poplawski <jarkao2@...il.com> wrote:
> 
> > diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
> > index d304d38..f6ecad8 100644
> > --- a/drivers/net/netconsole.c
> > +++ b/drivers/net/netconsole.c
> > @@ -702,6 +702,8 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
> >  	if (list_empty(&target_list))
> >  		return;
> >  
> > +	/* Avoid enabling softirqs with hardirqs disabled */
> > +	local_bh_disable();
> >  	spin_lock_irqsave(&target_list_lock, flags);
> >  	list_for_each_entry(nt, &target_list, list) {
> >  		netconsole_target_get(nt);
> > @@ -723,6 +725,7 @@ static void write_msg(struct console *con, const char *msg, unsigned int len)
> >  		netconsole_target_put(nt);
> >  	}
> >  	spin_unlock_irqrestore(&target_list_lock, flags);
> > +	local_bh_enable();
> 
> but netconsole can be triggered from printk - and printk can be called 
> from hardirqs-off sections - so this doesnt really fix the bug. 
> Netconsole should not do BH processing.

Well, it sort of "has to".  It calls the NAPI ->poll() method of the
driver to try and make forward progress with TX reclaim so it can
send new messages.

It is very careful not to recursively invoke into ->poll() and other
nasty situations.

Didn't you write some of this code Ingo a very long time ago? :-)))

Anyways, I'll look more closely at this and the original report,
this never was a problem before.
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