linux-kernel - Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

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Date:	Wed, 13 Apr 2011 10:41:57 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>, "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

On Wed, 2011-04-13 at 10:32 +0800, Eric Dumazet wrote:
> Le mercredi 13 avril 2011 à 09:01 +0800, Shaohua Li a écrit :
> > On Tue, 2011-04-12 at 17:03 +0800, Eric Dumazet wrote:
> > > 
> > > Hmm... did you test this with LOCKDEP on ?
> > > 
> > > You add a possible deadlock here.
> > > 
> > > Hint : Some percpu_counter are used from irq context.
> > there are some places we didn't disable interrupt, for example
> > percpu_counter_add. So the API isn't irq safe to me.
> > 
> 
> So what ? Callers must disable IRQ before calling percpu_counter_add(),
> and they actually do in network stack. Please check again,
> tcp_sockets_allocated for example.
Did you check other code? for example, __vm_enough_memory() doesn't
disable IRQ before calling percpu_counter_add().

> > > This interface assumes caller take the appropriate locking.
> > no comments say this, and some places we don't hold locking.
> > for example, meminfo_proc_show.
> > 
> 
> This doesnt answer my question about LOCKDEP ;)
> 
> Just fix the few callers that might need a fix, since this is the only
> way to deal with potential problems without adding performance penalty
> (for stable trees)
I mean the interface doesn't assume caller should take locking. Since
there isn't locking taking, we should make the interface itself correct,
instead of fixing caller.

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